Prepare the Cookie Dough – Preheat the oven to 350°F (175°C) and grease a round cake pan. In a large mixing bowl, cream together butter, brown sugar, and granulated sugar until light and fluffy. Add the egg and vanilla extract and mix until combined.
Add the Dry Ingredients – In a separate bowl, whisk together flour, baking soda, and salt. Gradually mix the dry ingredients into the wet ingredients until fully incorporated. Fold in the chocolate chips, M&Ms, and sprinkles.
Bake the Cookie Cake – Press the dough evenly into the prepared cake pan. Bake for 18-22 minutes or until golden brown. Let it cool completely before decorating.
Make the Frosting – Beat powdered sugar, softened butter, vanilla extract, and milk together until smooth and creamy.
Decorate & Serve – Spread frosting over the cooled cookie cake, then sprinkle extra M&Ms and pastel sprinkles on top. Slice and enjoy!