Mix flour, sugar, and salt in a large bowl. Set aside.
Heat butter, water, and cocoa powder in a sauce pan over medium heat. Stir as butter melts. Bring to a boil then remove from heat.
Slowly pour the butter/chocolate mixture over the dry ingredients. Stir.
Mix eggs, milk, vinegar, baking soda and vanilla together in a small bowl. Add to the large bowl and mix until combined.
Pour batter into a greased 12" x 17" baking sheet.
Bake for 25 minutes. Let cool.
Cream butter and peanut butter in a large bowl. Add sugar, vanilla and milk. Beat on high until fluffy.
Spread over cooled cake.
Note: Mini Reese's cups, Reese's Pieces, a mixture of peanut butter chips and chocolate chips, or any other chocolate peanut butter combination would be delicious if you don't have mini Buckeye candies.