Peanut Butter Bars: Cream the butter, peanut butter, sugars and eggs together until smooth. Add the remaining ingredients and beat until incorporated.
Spread into a greased cookie sheet (jelly roll size) until even on all sides (batter will be thick, you may need to use a fork to press down).
Bake at 325 for 13 minutes (bar will seem under done, but will firm up as it sits). This recipe makes a large batch, it can be halved and put in a 9×13 pan if you would prefer.
Chocolate Frosting: Beat the butter, peanut butter, cocoa powder, and milk together until smooth and soft.
Add the powdered sugar, salt, and vanilla and continue to beat until fluffy and smooth. Spread a thin layer of frosting over the cooled peanut butter bars.