Preparation Steps

• Preheat the oven to 350°F. Line a 9-inch cake pan with parchment paper and grease the sides.
• In a bowl, cream together butter, peanut butter, brown sugar, and sugar until light and fluffy, about 3-4 minutes.
• Add the egg and vanilla extract to the mixture and blend until combined.
• In a separate bowl, whisk together flour, baking soda, baking powder, and salt.
• Gradually add the dry ingredients to the wet ingredients and mix until fully incorporated.
• Stir in the oats, followed by M&Ms and chocolate chips.
• Press the thick cookie dough evenly into the prepared pan, pressing some extra mix-ins into the surface.
• Bake for 16 to 18 minutes or until edges are golden brown. Let cool completely in the pan.
• To make the frosting, beat butter until smooth. Gradually add half the powdered sugar and mix well.
• Add cocoa powder, vanilla extract, 1 tablespoon of water, and remaining powdered sugar; mix until smooth. Add more water as needed.
• Pipe or spread frosting onto the edges of the cooled cookie cake. Store in an airtight container until serving.