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

In a large bowl, cream together the softened butter, peanut butter, brown sugar, and granulated sugar until the mixture is light and fluffy, about 3-4 minutes using an electric mixer.

Beat in the egg and vanilla extract until they are well combined with the butter mixture.

In a separate medium bowl, whisk together the all-purpose flour, baking soda, baking powder, and salt.

Gradually add the dry ingredients to the wet butter mixture, mixing until just combined. Be careful not to overmix.

Stir in the quick cook oats. The cookie dough will be quite thick.

Fold in the M&Ms and chocolate chips. They might not fully stick to the dough, but they will integrate during baking.

Press the cookie dough evenly into the prepared 9-inch cake pan. You may need to press some of the mix-ins into the dough.

Bake for 16-18 minutes, or until the edges of the cookie cake are slightly golden brown.

Remove the cookie cake from the oven and allow it to cool completely in the pan before frosting.

While the cookie cake cools, prepare the frosting. In a clean bowl, beat the softened butter until it is smooth and creamy.

Gradually add about half of the powdered sugar to the butter, mixing until well combined and smooth.

Add the cocoa powder, vanilla extract, 1 tablespoon of water (or milk), and the remaining powdered sugar. Mix until the frosting is smooth and free of lumps.

If the frosting is too thick, add more water or milk, 1 teaspoon at a time, until you reach your desired consistency for piping.

Once the cookie cake is completely cool, pipe the frosting onto the edges of the cookie cake or spread it over the entire surface. Store the finished cake in an airtight container until ready to serve.