Transfer batter to prepared pan and spread evenly.
Bake 20 to 25 minutes, or until a pick inserted into the center of the cake comes out clean.
Cool in pan on wire rack for 10 minutes. Then, spread the peanut butter over the cake. Allow to cool completely.
To make the icing, place the sifted confectioners’ sugar in a large bowl. Set aside.
Combine butter, buttermilk, and cocoa in a small saucepan. Cook over medium heat, stirring constantly, until the mixture begins to boil.
Remove from heat and pour over confectioners’ sugar. Stir until smooth. Stir in vanilla.
Spread icing over peanut butter. The cake can be served immediately or can sit at room temperature to allow the icing to set
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