Peanut Butter Fudge Mousse

1 devil's food cake mix
1/2 cup melted butter
1/4 cup milk
1 egg
3/4 cup peanuts

Filling
3/4 cup peanut butter
1 1/2 cups powdered sugar
8 ounces softened cream cheese
2 1/2 cups milk
1 (8 ounce) container Cool Whip