Preheat oven to 350°F. Grease a 9-inch round cake pan or springform pan and line the bottom with parchment paper.
In a large bowl, beat butter and sugars until light and fluffy. Add the egg and vanilla and mix well.
Stir in flour, baking soda, and salt until just combined.
Fold in chocolate chips and M&Ms.
Press the dough evenly into the prepared pan and sprinkle with extra chocolate chips and M&Ms if desired.
Bake for 20–25 minutes, or until the edges are golden brown and the center is set.
Let cool completely before slicing. For cleaner slices, chill slightly before cutting.