Preheat Oven & Prep Pan

Preheat your oven to 350°F (177°C).

Line a 9×13-inch metal or glass baking pan with parchment paper, leaving an overhang on the sides to easily lift the bars out after baking. Set aside.

Combine Dry Ingredients

In a large bowl, whisk together:

Flour, cornstarch, baking soda, and salt.

Set aside.

Mix Wet Ingredients

In a separate medium bowl, whisk together:

Melted butter, brown sugar, and granulated sugar until smooth and lump-free.

Add the eggs and vanilla extract. Whisk until fully combined.

Combine and Fold in Add-ins

Pour the wet mixture into the dry ingredients. Stir with a silicone spatula or wooden spoon until a thick, slick dough forms.

Fold in the M&Ms and chocolate chips. (They may not stick well due to the melted butter texture—this is normal.)

Spread and Bake

Transfer the dough to your prepared baking pan.

Use a spatula or clean hands to press and smooth the dough evenly into all corners.

Bake for 26–30 minutes, or until the edges are lightly browned and a toothpick inserted in the center comes out with a few moist crumbs (not wet batter).

If the bars begin to brown too quickly before 25 minutes, loosely tent with aluminum foil.

Cool and Slice

Once baked, remove from the oven and allow to cool in the pan on a wire rack for at least 1 hour.

While still warm, you can gently press additional M&Ms or chocolate chips on top for visual appeal (optional).

Lift the bars out using the parchment overhang, transfer to a cutting board, and slice into squares.