Prep the Oven: Preheat your oven to 350°F and line two large baking sheets with parchment paper.

Brownie Dough:

• Cream butter and brown sugar until light and fluffy (about 3 minutes).

• Add the egg and vanilla, mixing until fully combined.

• In a separate bowl, sift together the flour, cocoa powder, baking soda, and salt.

• Gradually mix the dry ingredients into the butter mixture on low speed until fully incorporated.

• Fold in chocolate chips. Set aside.

Chocolate Chip Dough:

• Beat cold, cubed butter with both sugars until pale and creamy (about 4 minutes).

• Add the egg and egg yolk one at a time, mixing and scraping the bowl between additions.

• Stir in vanilla.

• Combine flour, cornstarch, baking soda, and salt in another bowl, then mix into the wet ingredients in two additions.

• Fold in the chocolate chips without over-mixing.

Shape the Dough:

• Scoop slightly larger balls of chocolate chip dough and slightly smaller balls of brownie dough.

• Press them together and roll into a single ball.

• Place on baking sheets 2 inches apart.

Bake:

• Bake one sheet at a time for 8 to 9 minutes, until edges are set but centers are slightly underdone.

• Cool on the baking sheet for 5 to 10 minutes, then transfer to a wire rack.

• Sprinkle with flaky sea salt if desired.