Pre heat oven to 350°F.
Bake cake according to box directions. Using a 9x13 baking dish.
Heat butter and milk in heavy saucepan.
Bring to a boil and stir constantly. Boil 2 minutes.
Remove from heat and add peanut butter and powdered sugar. Stir until smooth, over low heat.
It should start to turn glossy. Remove from heat and add vanilla.
Pour quickly over hot cake. Spread evenly and let cool.
After you add powdered sugar, if it looks to dry, just add a small amount of milk to loosen it up. A little goes a long way, so I use 1/8 of a teaspoon to start.