In a large saucepan melt butter over medium heat. Add water and peanut butter and bring to a boil. Remove from heat and stir in sugar. To the saucepan add flour, baking soda, and salt. Stir until well blended.
In a small bowl whisk together eggs, milk, and vanilla extract. Add liquids to saucepan and stir until very well combined and smooth. Pour into prepared pan and bake 23-25 minutes, rotating pan once during baking. Cake is done when a toothpick inserted in middle comes clean. (For 9X13" pan see recipe notes). As soon as cake comes out of oven, make chocolate frosting.
In a medium saucepan add butter and cocoa powder. Stir together over medium heat until butter melts and mixture is very smooth.
Remove from heat and stir in sour cream and vanilla extract. Gradually stir in powdered sugar until frosting is smooth and no lumps remain. Immediately pour on warm cake and spread evenly. Sprinkle on chopped peanuts. Eat warm to experience cake heaven.
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Tips for Making Chocolate Peanut Butter Cake
This cake recipe can also be baked in a 9X13 pan, although I prefer the thinner cake to frosting ratio. It will take 35-40 minutes to bake in 9X13 pan.
Substitute chunky peanut butter in the cake.
Using natural peanut butter is not recommended for this recipe, but if that's all you have on hand make sure the oil is incorporated well before measuring out.
As soon as the peanut butter cake comes out of the oven, make the easy stovetop chocolate frosting. It's best spread on a still warm cake.
Yogurt can be substituted for the sour cream in the frosting