Chocolate Peanut Butter Cake

1 (18.25 ounce) box Duncan Hines yellow cake mix

Frosting:
1 1/3 cups granulated sugar
2 tablespoons (1/4 stick) butter or margarine
3 tablespoons cocoa
1 small can evaporated milk
1 teaspoon vanilla extract
1 whole egg
1 cup peanut butter (or to taste)

Prepare cake batter in two layers and then split them horizontally to make 4 layers.

Blend butter and sugar. Blend in cocoa. Add milk, beaten egg and vanilla extract. Cook, stirring constantly until it boils. Boil on low for 2 minutes. Add peanut butter to taste. If grainy, add more milk, stirring well. Spread filling on layers to cover tops and sides.