In this article
In my family the peanut butter sheet cake was always gone before anybody touched the chocolate one, and I have watched that happen at every reunion for forty years. There would be two long pans on the dessert table, side by side, and by the time the men had finished their second plate of supper the peanut butter pan was scraped clean and the chocolate pan was still half full. My brothers cut their squares from the corners, because the corners carry the most frosting, and my mother would pretend not to notice.
This is the same kind of cake as the Texas sheet cake: a thin, tender buttermilk cake mixed in one bowl and one saucepan, baked fast in a big shallow pan. The difference is the peanut butter, which is melted right into the boiling butter and water for the batter, and then melted again into the frosting. The pan size is what makes it work. In a 15x10 inch jelly roll pan the cake bakes up thin, about an inch tall, so it cooks through in about twenty minutes and stays moist from edge to edge. And the frosting goes on the second the cake comes out of the oven, so it melts into the top before it sets.
Why this peanut butter sheet cake works
- Boiled butter, water and peanut butter. Pouring the hot mixture over the flour and sugar blooms the peanut butter and gives the cake its soft, fine crumb.
- Buttermilk. It keeps the cake tender and adds a little tang that stops the peanut butter from tasting flat.
- A thin cake in a big pan. The 15x10 inch pan gives a cake about an inch tall that bakes quickly and evenly, with no dry edges.
- Frosting poured on a hot cake. The warm frosting melts into the top of the hot cake and sets into a smooth, fudgy layer as both cool together.
- One saucepan for everything. The batter base and the frosting are both made in the same pan, so cleanup is one pot and one bowl.
Ingredients
Makes one 15x10 inch sheet cake (a jelly roll pan), about 24 squares. The measurements are in US cups with grams in brackets.
For the cake
- 2 cups all-purpose flour (250g)
- 2 cups granulated sugar (400g)
- 1 teaspoon baking soda
- 1/2 teaspoon fine salt
- 1 cup unsalted butter (226g)
- 1 cup water (240ml)
- 1/2 cup creamy peanut butter (130g)
- 1/2 cup buttermilk (120ml)
- 2 large eggs, beaten
- 1 teaspoon vanilla extract
For the peanut butter frosting
- 1/2 cup unsalted butter (113g)
- 1/2 cup creamy peanut butter (130g)
- 1/3 cup whole milk (80ml)
- 1 teaspoon vanilla extract
- 3 1/2 cups powdered sugar (420g), sifted
Use an ordinary creamy peanut butter, the kind that does not separate in the jar. Natural peanut butter with oil on top makes a greasy frosting that will not set properly.
How to make peanut butter sheet cake
- Heat the oven to 350 degrees (175°C). Grease a 15x10 inch jelly roll pan, the kind with a one inch rim.
- In a large bowl, whisk the flour, sugar, baking soda and salt together.
- In a saucepan over medium heat, bring the butter, water and peanut butter to a boil, stirring until the peanut butter melts into it. Pour the hot mixture over the flour and sugar and stir until smooth.
- Stir in the buttermilk, eggs and vanilla. The batter will be thin, about like pancake batter. That is right.
- Pour the batter into the pan and tip it so it runs into the corners. Bake at 350 degrees for 20 to 22 minutes, until the top springs back when you touch it and a toothpick in the center comes out clean.
- While the cake bakes, rinse the saucepan and make the frosting: bring the butter, peanut butter and milk just to a bubble over medium heat, stirring. Take it off the heat, stir in the vanilla and beat in the powdered sugar until smooth and glossy.
- As soon as the cake comes out of the oven, pour the warm frosting over the hot cake and spread it quickly to the edges with the back of a spoon. It will melt into the top.
- Let the cake cool in the pan for at least 30 minutes, until the frosting sets, then cut it into squares.
Tips that make the difference
- Make the frosting while the cake bakes. It has to be warm when the cake comes out, and it only takes five minutes.
- Do not overbake. A thin sheet cake goes from moist to dry in a couple of minutes, so start checking at 20 minutes.
- Sift the powdered sugar. Lumps will not melt out of a frosting that sets this fast.
- If you only have a 13x9 inch pan, the cake will be thicker: bake it 30 to 35 minutes and expect a softer, cakier square.
Variations
- Crunchy top. Scatter 1/2 cup chopped salted peanuts over the wet frosting, the way my uncle liked it.
- Chocolate frosting. Use the peanut butter cake with a chocolate sheet cake frosting for the best of both pans.
- Peanut butter and jam. Swirl a few spoonfuls of warm strawberry jam through the frosting before it sets.
- Half sheet. Double nothing; the same recipe fits an 18x13 inch half sheet pan for a thinner cake, baked 16 to 18 minutes.
Storage and make-ahead
Keep the cake in its pan, covered with foil, at room temperature for up to 3 days. The frosting stays soft and fudgy.
In a warm kitchen, keep it in the refrigerator and let squares sit out 20 minutes before serving.
It freezes well: wrap cut squares in plastic and foil for up to 2 months and thaw them on the counter, still wrapped.
Frequently asked questions
What size pan do I need for peanut butter sheet cake?
A 15x10 inch jelly roll pan with a one inch rim. That is what keeps the cake thin and quick to bake. A 13x9 inch pan works but gives a thicker cake that needs 30 to 35 minutes.
Why do you pour the frosting on a hot cake?
So it melts into the top and sets into a thin fudge layer. Frosting a cold sheet cake gives you a separate layer that sits on top instead of becoming part of the cake.
Can I use crunchy peanut butter?
In the frosting, yes, if you like the bits. In the batter, creamy is better because it melts smoothly into the boiling butter.
Can I make it without buttermilk?
Stir 1/2 tablespoon of lemon juice into 1/2 cup of milk and let it stand five minutes. It is not quite the same, but it is close.
Bake two pans if you can. One for the table, and one for the people who always ask whether there is any more of the peanut butter one.
Peanut Butter Sheet Cake with Warm Peanut Butter Frosting
The old-fashioned peanut butter sheet cake: a thin buttermilk cake made with boiled butter, water and peanut butter, baked in a 15x10 inch pan at 350 degrees for 20 to 22 minutes, with a warm peanut butter frosting poured over the hot cake.
Ingredients
- 2 cups all-purpose flour
- 2 cups granulated sugar
- 1 teaspoon baking soda
- 1/2 teaspoon fine salt
- 1 cup unsalted butter
- 1 cup water
- 1/2 cup creamy peanut butter
- 1/2 cup buttermilk
- 2 large eggs, beaten
- 1 teaspoon vanilla extract
- 1/2 cup unsalted butter
- 1/2 cup creamy peanut butter
- 1/3 cup whole milk
- 1 teaspoon vanilla extract
- 3 1/2 cups powdered sugar (420g), sifted
Instructions
- Heat the oven to 350 degrees (175°C). Grease a 15x10 inch jelly roll pan, the kind with a one inch rim.
- In a large bowl, whisk the flour, sugar, baking soda and salt together.
- In a saucepan over medium heat, bring the butter, water and peanut butter to a boil, stirring until the peanut butter melts into it. Pour the hot mixture over the flour and sugar and stir until smooth.
- Stir in the buttermilk, eggs and vanilla. The batter will be thin, about like pancake batter. That is right.
- Pour the batter into the pan and tip it so it runs into the corners. Bake at 350 degrees for 20 to 22 minutes, until the top springs back when you touch it and a toothpick in the center comes out clean.
- While the cake bakes, rinse the saucepan and make the frosting: bring the butter, peanut butter and milk just to a bubble over medium heat, stirring. Take it off the heat, stir in the vanilla and beat in the powdered sugar until smooth and glossy.
- As soon as the cake comes out of the oven, pour the warm frosting over the hot cake and spread it quickly to the edges with the back of a spoon. It will melt into the top.
- Let the cake cool in the pan for at least 30 minutes, until the frosting sets, then cut it into squares.
