Preheat your oven to 325°F (163°C) and lightly spray a 9" x 13" baking pan with cooking spray or line it with foil.

In a medium bowl, whisk together 1 ½ cups of all-purpose flour, ½ teaspoon of Morton kosher salt, and ½ teaspoon of baking soda until combined.

In a larger bowl, whisk together ¾ cup of melted unsalted butter with ½ cup of light brown sugar and ½ cup of granulated sugar until smooth.

Add 1 egg, 1 egg yolk, and 1 tablespoon of pure vanilla extract to the wet mixture, whisking until fully incorporated.

Gradually fold the dry mixture into the wet ingredients until just combined, stirring in 1 cup of semi-sweet chocolate chips halfway through.

Spread the dough evenly in your prepared baking pan, pressing down firmly with a spatula.

Optional: sprinkle additional chocolate chips or M&M's on top of the dough before baking.

Bake in the preheated oven for 23 minutes, checking that the edges are golden-brown and the center remains soft.

Remove from oven, sprinkle with sea salt flakes, and cool in the pan for about 10 minutes before slicing into squares.