01 -
Preheat the oven to 350°F. Line a 9x13-inch metal baking pan with parchment paper and lightly grease the exposed sides.
02 -
In a large bowl, whisk the melted butter, brown sugar, and granulated sugar until smooth and glossy. Whisk in the eggs, egg yolk, and vanilla until fully combined.
03 -
Add the flour, cornstarch, baking soda, and salt. Stir with a spatula just until the dough comes together and no dry flour remains.
04 -
Fold in 3/4 cup of the M&M's and 1/2 cup of the chocolate chips. Reserve the rest for topping.
05 -
Spread the thick dough evenly in the prepared pan. Scatter the reserved chocolate chips and a few reserved M&M's over the top, pressing them in lightly.
06 -
Bake for 24-28 minutes, until the edges are golden and the center is set with a few moist crumbs on a toothpick. Do not overbake.
07 -
Press the remaining M&M's onto the warm bars for a bright top. Cool completely in the pan, then lift out with the parchment and slice into 24 bars.