1-Preheat your oven to 350°F (175°C). Line a 9×13 inch baking pan with foil or parchment paper, leaving an overhang for easy removal, and lightly spray or grease the bottom.

2-In a bowl, whisk together the flour, baking soda, salt, and cornstarch until combined.

3-In a large mixing bowl, blend melted butter with brown sugar and granulated sugar until smooth with no lumps.

4-Add eggs one at a time, mixing well after each addition, then stir in vanilla extract.

5-Gradually add the dry ingredients to the wet mixture, stirring just until incorporated to avoid overmixing.

6-Fold in chocolate chips, reserving some to sprinkle on top. If using toffee bits or nuts, fold them in now or sprinkle on top before baking.

7-Press the cookie dough evenly into the prepared pan with clean, slightly damp hands or a spatula. Optionally sprinkle reserved chocolate chips and flaky sea salt on top.

8-Bake for about 25-35 minutes, starting to check at 25 minutes, until edges are golden brown and the center is set but still soft avoid overbaking to keep bars chewy. A toothpick inserted near the center should come out mostly clean with a few moist crumbs.

9-Allow bars to cool completely in the pan at room temperature (about 1 hour), or chill to achieve a firmer texture before cutting into squares.