I used the boxed cake mix for the cake. It made a dozen cupcakes and a small bake pan, I think 8in X 8in.
Prepare as directed. Then drop peanut M&M's in the bottom of the batter. Swirl the batter till they're covered. The pans should be coated in spray oil and flour.
Here's where I ran into some trouble... The M&M's sink to the bottom, and then sort of melt into nothing. This left the bottom of the cake super moist. and the cupcakes fell apart. I did some of them with cupcake liners, and those came out fine.
I used the peanut butter frosting recipe from The Brown Eyed Baker, and the chocolate frosting recipe from Wilton.
Put 1 cup confectioners’ sugar, 1 cup creamy peanut butter, 5 tbsp unsalted butter (at room temperature), 3/4 tsp vanilla extract, and 1/4 tsp kosher salt in a bowl with an electric mixer. Mix together until well incorporated. Then add 1/3 cup heavy cream and mix until light and fluffy.
Frost the cupcakes in the peanut butter frosting. Dump out the cake from it's pan and frost half of the cake in peanut butter frosting.
Cut the cake in half and put the layer on top of the other one. Dump even more peanut butter on the top and frost the sides as well.
Now for the chocolate frosting. I didn't take any pictures of the frosting for some reason. So here's the recipe: beat 1/2 cup solid vegetable shortening and 1/2 cup (1 stick) butter, softened together until light and fluffy. Add 3/4 cup dark chocolate cocoa powder and 1 tsp vanilla extract, mix until incorporated. Add 4 cups confectioner's sugar, one cup at a time. Add 3 to 4 tbsp milk, adding one at a time until desired consistence is reached. Frost the cupcakes and the cake.
Add some peanut M&M's on top to decorate.