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Grab your small baking pan, because we have another recipe to make with it! This Peanut Butter Sheet Cake has a rich frosting that is similar to fudge plus a soft, tender cake underneath. It’s the best use for that quarter sheet cake pan you have!

Overhead shot of peanut butter sheet cake cut into 12 equal squares.

I am the last person on earth to join one of those grocery store clubs that sells in bulk (I don’t think I can say the name on here, but you know what I mean). Anyway, they had a 3-pack of quarter sheet pans for a really good price this weekend, and it made me realize that we haven’t baked a small cake in a while!

My Texas chocolate sheet cake in a quarter sheet pan has become a favorite, and so I know you’ll love the peanut butter version of it. If you want to mix and match the frostings for these recipes, please do! A chocolate cake with peanut butter fudge frosting sounds just as good as a peanut butter cake with chocolate frosting!

Square slice of peanut butter sheet cake on white plate with fork.

Ingredients

Small white bowls of flour, sugar, and powdered sugar on white counter.

Flour. This recipe is in metric and standard cups for you. You need one and a quarter cups (156 grams) of all-purpose flour.
Sugar. One cup (200 grams) of granulated sugar.
Salt. One-quarter teaspoon of salt.
Cinnamon. A small amount of cinnamon (1/4 teaspoon) subtly brings out the peanut butter flavor.
Baking Soda. One-half teaspoon of baking soda.
Butter. We need 5 tablespoons (70 grams) of unsalted butter for the cake and an additional 6 tablespoons (85 grams) for the frosting.
Peanut Butter. Just 3 tablespoons of peanut butter for the cake and 1/4 cup of peanut butter for the frosting.
Buttermilk. You can use one-half cup of low-fat buttermilk for this cake, or you can use 1/2 cup of whole milk with one-half teaspoon of lemon juice added to it, if you don’t have any buttermilk.
Egg. One large egg for the cake mixture.
Powdered Sugar. The frosting needs 2 cups of powdered sugar.
Vanilla Extract. One-half teaspoon of real vanilla extract for the frosting. The cake part does not contain any vanilla so that it doesn’t overpower the peanut flavors.

How to Make Peanut Butter Sheet Cake

Preheat the oven to 350-degrees F, and lightly spray a 9x13x1-inch quarter sheet pan with cooking spray.

Cream colored bowl with flour, sugar and cinnamon.

Clear glass bowl with melted butter and brown mixture.

2. In a large bowl, whisk together the flour, sugar, salt, cinnamon, and baking soda.

3. In a microwave-safe bowl, combine the butter and peanut butter. Heat for 30-second intervals until melted. Whisk together until smooth, and set aside.

Clear glass bowl with melted brown liquid, egg and milk.

4. Add the buttermilk and egg to the melted butter mixture, and whisk to combine.

5. Pour the wet ingredients over the dry, mix gently to combine, and then scrape the mixture into the prepared quarter sheet pan.

Freshly baked peanut butter sheet cake on marble surface.

Small saucepan with powdered sugar, butter and peanut butter.

6. Bake for 22-26 minutes, until a toothpick inserted in the center comes out with only moist crumbs. The edges of the cake will not pull away from the pan, fyi.

7. While the cake is still warm, begin making the frosting: add the butter, peanut butter, buttermilk, and powdered sugar in a small sauce pan. Heat over low until the frosting is melted and pour-able. Stir in the vanilla’s nd turn off the heat.

Runny brown frosting being poured on golden brown rectangular cake.

Square slices of brown cake garnished with crushed nuts.

8. Pour the warm frosting over the cake, spread to the edges and let set.

9. Slice the cake into 12 equal slices and serve.

Storing Leftovers:

This peanut butter sheet cake keeps well at room temperature for up to 2 days. You can move it to the fridge for an extra few days. I don’t recommend freezing it.

Thick slice of peanut butter cake with peanut butter fudge frosting.

This is the quarter sheet pan I use and love. I own 3 of them!

Other Quarter Sheet Cakes:

Red Velvet Quarter Sheet Cake with cream cheese frosting.
Banana Sheet Cake also with cream cheese frosting!
Pumpkin Sheet Cake with warm cinnamon frosting.