Preheat the Oven: Preheat your oven to 350°F (175°C) and line a 12-cup muffin pan with cupcake liners.
Mix the Wet Ingredients: In a stand mixer, cream together butter and sugar for about 3 minutes until light and fluffy. Add peanut butter and mix until smooth. Add eggs, vanilla, and milk, mixing until fully combined.
Combine the Dry Ingredients: Sift together the flour and baking powder. Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not over-mix.
Add the Mix-Ins: Stir in chocolate chips and chopped peanuts.
Scoop and Bake: Use a 1/4 cup ice cream scoop to portion the batter into the cupcake liners. Bake for 20 minutes, or until a toothpick inserted comes out clean. Let the cupcakes cool completely on a wire rack.
Make the Frosting: For the frosting, cream together the butter, cream cheese, and peanut butter until smooth. Add brown sugar, vanilla, oats, and flour, then mix in powdered sugar until smooth. Finally, fold in M&Ms and chocolate chips.
Frost the Cupcakes: Once the cupcakes have cooled, use a 1/4 cup ice cream scoop to top each with a generous amount of cookie dough frosting.