Preparation Steps

• Preheat the oven to 350°F. Line a 9-inch cake pan with parchment paper and grease the sides.
• Cream together 0.25 cup butter, peanut butter, brown sugar, and sugar until light and fluffy, about 3-4 minutes.
• Beat in the egg and vanilla extract until combined.
• In a separate bowl, whisk together flour, baking soda, baking powder, and salt.
• Add the dry ingredients to the wet mixture and mix until fully combined.
• Stir in the oats until well distributed throughout the dough.
• Fold in the M&Ms and chocolate chips; they may not fully stick to the dough but will stay once pressed into the pan.
• Press the cookie dough evenly into the prepared cake pan, pressing some mix-ins into the edges to secure them.
• Bake for 16-18 minutes until edges are golden. Remove from oven and allow to cool completely in the pan.
• To make the frosting, beat 0.5 cup butter until smooth. Gradually add half the powdered sugar and mix until combined.
• Add cocoa powder, vanilla extract, 1 tablespoon water, and the remaining powdered sugar; mix until smooth. Add more water if needed to reach desired consistency.
• Pipe the frosting around the edges of the cookie cake. Store in an airtight container until ready to serve.