In a small bowl, mix together flour, baking powder, baking soda, and salt. Set aside. In a large bowl with mixer on medium speed, beat butter, brown sugar, and sugar until light and fluffy. Add egg and vanilla and beat until combined. Add flour mixture (beating slowly at first) until just combined. With a spatula, fold in chocolate chips.
Preheat oven to 350 F. Grease a 10-inch round springform pan; evenly press dough to the edges. Sprinkle with M&Ms, lightly pressing them into dough. Bake 18 to 20 minutes, until golden brown. Cool to room temperature on a rack. Meanwhile, make the frosting: Beat butter until fluffy, then mix in powdered sugar and vanilla until smooth. Add a bit of milk or cream for ease of piping.
Run a butter knife along the inside edge of the springform pan to loosen. Unlock springform and remove the side of the pan. Pipe frosting around the edges. Place cookie in an airtight container in the fridge. Chilled cookie should be easy to remove from springform base. Slice into wedges to serve.
*To prevent buttercream from melting on hot summer days, increase the amount of powdered sugar or mix in a tablespoon of meringue powder or cornstarch, and a bit of milk or cream for easier piping.
Per serving: 324 calories, 15 grams fat (9 grams saturated fat), 46 grams total carbohydrates, 45 milligrams cholesterol, 115 milligrams sodium, 1 gram fiber, 3 grams protein.
