Step 1: Prepare Pan

Position your oven rack in the lower-middle position to provide even heat. Preheat oven to 325°F, which provides gentle, even baking that prevents over-browning while providing perfectly cooked. Lightly mist a 9″x13″ metal baking pan with non-stick spray, or line with parchment paper or aluminum foil, leaving some material to drape over long sides to make easy removal. When lining, also mist liners so that they’ll release easily and provide neat, professional-cutting appearance.

Step 2: Combine Dry Ingredients

Whisk the flour, kosher salt, and baking soda together in a medium bowl until well combined. This initial combination guarantees leavening agents and flavorings are evenly dispersed throughout the final dough to avoid inconsistencies in texture or pockets of flavor. Reserve this blend for the time when combining wet ingredients.

Step 3: Incorporate Wet Ingredients

In a large mixing bowl, beat together room temperature melted butter that has cooled, light brown sugar that is packed, and granulated sugar until the mixture is of suave, blended consistency. The butter has to be fully incorporated with the sugars so as to impart a shiny base. Whisk the whole egg, the egg yolk, and vanilla extract together well until the mixture ends up being extremely smooth and uniform.

Step 4: Add Chocolate and Mix

Using a firm spatula, gently stir in the prepared dry ingredients into the wet mixture. At the halfway point in the flour mixture incorporation, add the chocolate chips and fold until just combined. The secret here is to be gentle – don’t over-mix, which will begin to form gluten and result in tough, dense bars instead of the best tender texture.

Step 5: Press into Pan

Transfer your sticky, heavy dough to your prepared-to-use baking pan. Press dough flat onto the entire surface of the pan with a spatula or clean fingers. Pro tip: Use the butter side of your butter wrapper or lightly butter your fingers to avoid sticking and achieve an even, smooth texture without fighting with sticky dough.

Step 6: Top and Bake

Sprinkle more chocolate chips on the top if desired, pressing them gently into the dough so they stick during baking. For a fun twist, sprinkle on top mini M&M’s for added color and crunch. Bake at 23-25 minutes, watching closely – the top should be lightly golden and feel just firm to light pressure. Remember, for chewy bars perfection, slight under-baking is better than over-baking.

Step 7: Cool and Serve

Transfer the pan at once to a wire cooling rack. If using optional sea salt flakes, sprinkle over the hot surface immediately after removing from the oven – the heat remaining will cause the salt to stick nicely. Let the bars cool completely to room temperature before cutting. Use the foil or parchment overhang to lift the sheet out of the pan, onto a cutting board, and cut into equal squares with a sharp knife.