½cup1 stick unsalted butter, room temperature (plus more for the skillet)
½cuppacked light brown sugar
½cupsemisweet or white chocolate chips
Preheat the oven to 350°. Grease the bottom and sides of a 9-inch cast-iron skillet with 2 tablespoons of butter.
In a large bowl, whisk together the flour, baking soda, and salt. Set aside.
In the bowl of an electric mixer, beat ½ cup butter, sugar, and brown sugar, on medium speed, until well blended. Beat in the egg and vanilla. Add flour mixture; mix on low just until combine. Stir in chocolate chips.
Press the cookie dough into the prepared skillet. Sprinkle M&M’s over the top and gently press in to the dough. Sprinkle confetti sprinkles on top.
Bake for 20 to 25 minutes, until the edges are crisp and the cookie is golden and cooked through. Remove from oven. If desired, serve warm topped with vanilla ice cream.