Preheat oven to 350 degrees.
In a large bowl, combine the cake mix, 1/2 cup of butter, eggs, and peanut butter.
Reserve 1/3 cup of the dough, and press the remaining mixture into the bottom of a 13 x 9 inch pan.
In a microwave safe dish, combine 2 tablespoons of margarine, chocolate chips, and sweetened condensed milk.
Microwave on high until the chocolate chips are melted.
Fold in the remaining ingredients, and pour the chocolate mixture over the dough.
Crumble the reserved dough over the top.
Bake for 20 to 25 minutes.