Preheat oven to 350°F (176°C). Line the bottom of a 9-inch cake pan with parchment paper and grease the sides.

Cream together the butter, peanut butter, and sugars until light and fluffy, about 3-4 minutes.

Mix in the egg and vanilla extract until combined.

In a separate bowl, whisk together the flour, baking soda, baking powder, and salt.

Add the dry ingredients to the butter mixture and mix until well combined.

Stir in the oats. The cookie dough will be thick.

Stir in the M&Ms and chocolate chips. They may not stick to the dough completely, but will be fine once in the cake pan.

Pour the dough into the prepared cake pan and evenly press it into the bottom. Press some of the mix-ins that wouldn’t stick to the dough into the cake pan.

Bake for 16-18 minutes, or until the edges are slightly golden. Remove from oven and allow to cool completely in the pan.

To make the frosting, beat the butter until smooth.

Slowly add about half of the powdered sugar and mix until well combined.

Add the cocoa powder, vanilla extract, 1 tablespoon of water, and the remaining powdered sugar and mix until smooth.

Add more water, if needed, to get the right consistency.

Pipe the frosting onto the edges of the cookie cake, then store in an airtight container until ready to serve.