Prepare 15.25 ounce box chocolate cake mix along with ingredients needed to make cake mix; eggs, oil and water as directed on the back of the box for a 9x13-inch cake. Let it cool completely.
Once cake has cooled, begin making frosting. In a mixing bowl, combine 1/2 cup (1 stick) margarine, softened to room temperature with 3/4 cup Reese's creamy peanut butter. Mix until smooth.
Add in 2 cups powdered sugar, 2 Tablespoons milk and 1 teaspoon vanilla extract until combined.
Then start frosting the cake. Try not to yank it across your moist cake. Be soft handed and lightly spread.