Combine dry ingredients in large mixer bowl.
Add remaining ingredients; beat at medium speed 2 minutes (batter will be thin).
Pour into a greased and floured 13x9x2-inch pan.
Bake at 350° for 35 to 40 minutes or until cake tester comes out clean.
Cool completely and frost.
Cook the peanut butter, butter, and ¼-⅓ cup milk together over low heat until well incorporated. Simmer for about 15 seconds.
Pour into mixer. Beat in ½ cup cocoa.
Once well incorporated, alternate in ¼ cup milk with 2-3 cups sifted powdered sugar.
Add in 1 tsp vanilla. Beat with paddles well for several minutes.
