Ingredients
2 and 3/4 cups (343g) all-purpose flour (spooned and leveled): Provides structure without toughness; spooning prevents packing too densely, which keeps bars soft.
1 and 1/2 teaspoons cornstarch: Tenderizes the crumb like in chewy cookie recipes, absorbing moisture for a softer bake.
1 teaspoon baking soda: Gives lift and spread for thick bars; pairs with brown sugar acidity for chew.
1/2 teaspoon salt: Balances sweetness from sugars and chocolate.
1 cup (226g) unsalted butter, melted & cooled for 5 minutes: Melted creates fudgy texture; brief cooling prevents greasy dough or crumbly results.
1 cup (200g) packed light or dark brown sugar: Adds moisture, chew, and molasses depth; dark brown amps flavor.
1/2 cup (100g) granulated sugar: Sweetens lightly and aids crisp edges.
2 large eggs, at room temperature: Binds and adds tenderness; room temp whips in smoothly.
2 teaspoons pure vanilla extract: Rounds out flavors with warmth.
1 and 1/4 cups (260g) mini or regular-size M&Ms: Colorful crunch; minis distribute evenly, regulars add bigger bursts—press extras on top while warm.
3/4 cup (135g) semi-sweet chocolate chips: Melty pockets; use mini for subtlety or regular for gooeyness.
Instructions
Preheat oven to 350°F (177°C) and line a 9×13-inch metal or glass pan with parchment, overhanging sides for easy removal—metal conducts heat evenly for better browning. Whisk flour, cornstarch, baking soda, and salt in a large bowl; set aside to ensure even distribution.
In a medium bowl, whisk melted butter (cooled just 5 minutes), brown sugar, and granulated sugar until lump-free and glossy—brown sugar dissolves fully for smooth dough. Add room-temperature eggs and vanilla; whisk vigorously until combined and slightly thickened, about 1 minute.
Pour wet into dry ingredients; stir with a spatula or spoon until a thick, slick dough forms—avoid overmixing to keep texture soft. Fold in M&Ms and chocolate chips; they slip due to butter but distribute as baked—use mini M&Ms if sticking concerns you.
Transfer dough to prepared pan; press evenly with spatula or damp hands into corners, aiming 3/4-inch thick for uniform baking. Bake 26-30 minutes until edges brown lightly and center toothpick shows moist crumbs, not batter—tent loosely with foil after 25 minutes if browning fast. Cool in pan on wire rack 1 hour; press extra M&Ms/chips on warm top for looks. Lift out, slice into squares.