Peanut Butter Sheet Cake

​Cake:
1 cup water
½ cup butter
¼ cup creamy peanut butter
1 cup sugar
1 cup brown sugar
2 cups flour
1 teaspoon salt
1 teaspoon baking soda
2 eggs
½ cup buttermilk
1 teaspoon vanilla
 
Frosting:
¾ cup butter
1/3 cup buttermilk
1 teaspoon vanilla
½ cup peanut butter
3 ½ cups powdered sugar
 
1.  Preheat oven to 375 degrees and line a 12x17 pan with parchment paper trimmed to fit. 
 
2.  In a small pan, bring water to a simmer over medium-high.  Add butter and peanut butter and whisk until smooth and combined.  Bring just to a boil, then remove from heat and set aside to cool slightly.
 
3.  In a large bowl, stir together sugar, brown sugar, flour, salt and baking soda.  In a small bowl, whisk together eggs, buttermilk, and vanilla.  Use a rubber spatula to incorporate water/peanut butter mixture and egg mixture into dry ingredients.  Transfer to prepared pan and bake until edges are slightly browned and center springs back when touched, about 15-18 minutes.  Set on a wire rack to cool.
 
4.  While cake is baking, melt butter in a small pan over medium.  Add buttermilk, vanilla, and peanut butter and bring to a boil.  Transfer to a large bowl.  Sift powdered sugar into bowl and fold into peanut butter mixture until incorporated.  Beat with a wooden spoon until smooth and glossy.  Drizzle over warm cake and gently spread to coat with an offset spatula. 
 
Revised November 2017