In the bowl of a stand mixer, cream the butter and sugars on medium-high speed for about 3 minutes until light and fluffy.

Add the egg and vanilla, mixing to combine.

In a separate bowl, whisk together flour, cocoa powder, baking soda, and salt.

Sift the dry ingredients into the wet ingredients and mix on low until fully incorporated.

Fold in the chocolate chips. Set aside.

In a stand mixer, combine cold cubed butter, brown sugar, and granulated sugar. Beat until light and creamy (about 4 minutes).

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

Add the vanilla and mix.

In a separate bowl, whisk together flour, cornstarch, baking soda, and salt.

Add the dry ingredients to the butter mixture in two additions, mixing just until combined.

Fold in the chocolate chips. Do not overmix.

Preheat the oven to 350°F (175°C) and line baking sheets with parchment paper.

Using a small cookie scoop, portion out the dough—make brownie dough balls slightly smaller than the cookie dough balls.

Press a brownie ball and a cookie ball together, then roll them into one smooth ball.

Place dough balls 2 inches apart on prepared baking sheets.

Bake for 8–9 minutes, or until the edges are set and the tops are still slightly underbaked.

Let cool on the baking sheet for 5–10 minutes before transferring to a wire rack.

Optional: Sprinkle with flaky sea salt before serving.