Coarsely chop the candy bars.
Lightly grease and flour a 13x9 inch baking pan using butter or margarine.
Preheat the oven to 350.
Prepare the cake mix batter according to the package directions, except add the peanut butter, then mix.
Pour the batter into the prepared pan.
Bake for 30-40 minutes or until a wooden toothpick inserted in the center of the cake comes out dry.
Cool it completely in the pan on a wire rack.
Place the chopped candy bars and butter in a medium, heavy-duty saucepan.
Cook over low heat, stirring frequently, until it's all melted.
Let it cool.
Combine the evaporated milk, sugar, coconut, peanuts and egg yolks in a medium bowl; mix well.
Add this to the candy bar mixture.
Cook this over medium heat, stirring frequently, for about 10 minutes or until slightly thickened. Be careful with the heat so as to not cook the eggs.
Remove the pan from the heat to a bowl; cool it in the refrigerator for 30 minutes; stir.
Frost the cake with the candy bar frosting.
Refrigerate the cake until you're ready to serve it.