Preheat oven to 325 degrees.
Combine cake mix, melted butter, peanut butter, and eggs in a large bowl using a mixer or spoon. Press this into a 9X13 pan reserving 1 1/2 cup of the mixture to crumble on top.
In a small pot, melt chocolate chips, sweetened condensed milk, and butter. Remove from heat and stir in the vanilla (and coconut if using).
Spread chocolate mixture over the mixture pressed in the pan and then crumble the 1 1/2 cups of reserved cake mixture on top evenly.
Bake for 20-25 minutes. Cool completely before cutting into bars.