Simmer butter, water, buttermilk, and cocoa with Nesquick until thoroughly mixed.
In a large bowl, combine eggs, sugar, self-rising flour, baking soda and vanilla.
Pour cocoa mixture and peanut butter into the flour mixture.
Beat mixture until smooth.
Bake at 350 degrees for 30 minutes, or until a toothpick comes out clean. Let cool before adding icing.
Mix all icing ingredients until creamy. Spread on cake and serve.