Preheat and Prepare:Preheat your oven to 350°F (177°C). Line baking sheets with parchment paper or silicone baking mats.
Combine Dry Ingredients:In a medium-sized bowl, whisk together the flour, baking soda, cornstarch, and salt. Set aside.
Mix Wet Ingredients:In a large bowl, combine the melted butter, brown sugar, and granulated sugar. Beat until smooth and creamy. Add the egg, egg yolk, and vanilla extract, mixing until fully incorporated.
Combine Wet and Dry Ingredients:Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Avoid overmixing. Gently fold in the chocolate chips or chunks with a spatula.
Chill the Dough (Optional but Recommended):For thicker cookies, chill the dough for 30 minutes in the refrigerator.
Scoop and Bake:Scoop 1.5 tablespoons of dough for each cookie and place them 2 inches apart on the prepared baking sheets. Bake for 10-12 minutes, or until the edges are lightly golden but the centers look slightly underbaked.
Cool:Remove cookies from the oven and let them cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.
