Step 1: Brown the Butter

1. In a medium saucepan, melt the butter over medium heat.
2. Continue cooking, stirring frequently, until the butter turns golden brown and emits a nutty aroma (about 4–5 minutes).
3. Remove from heat and transfer to a heat-proof bowl to cool slightly.

Step 2: Mix the Wet Ingredients

1. In a large mixing bowl, combine the brown butter, dark brown sugar, and granulated sugar.
2. Whisk together until well incorporated.
3. Add the eggs and vanilla extract, then mix until smooth and creamy.

Step 3: Combine the Dry Ingredients

1. In a separate bowl, whisk together the flour, baking soda, salt, and cornstarch.
2. Gradually fold the dry ingredients into the wet mixture, stirring until just combined.

Step 4: Add the Chocolate

1. Fold in the semisweet chocolate chips and dark chocolate chunks until evenly distributed.

Step 5: Chill the Dough

1. Cover the dough and refrigerate for at least 30 minutes, or up to 24 hours for deeper flavor.

Step 6: Bake the Cookies

1. Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.
2. Scoop 1.5-inch balls of dough onto the baking sheet, spacing them about 2 inches apart.
3. Bake for 10–12 minutes, or until the edges are golden brown but the centers remain slightly soft.

Step 7: Cool and Enjoy

1. Let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack.
2. Sprinkle with flaky sea salt, if desired.
3. Enjoy warm or store for later!