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

In a mixing bowl, beat together the unsalted butter, light brown sugar, and granulated sugar until light and fluffy. Add one egg and a splash of vanilla extract, mixing until fully combined.

Gradually incorporate the Dutch-process cocoa powder, flour, baking soda, cornstarch, and salt, stirring just until a smooth batter forms. Fold in semi-sweet chocolate chips.

In another bowl, cream together more unsalted butter with granulated sugar and light brown sugar until fluffy. Add one egg and an additional egg yolk, mixing well.

Gently combine the all-purpose flour, baking soda, cornstarch, and salt into the wet ingredients until just combined. Stir in semi-sweet chocolate chips.

Roll small portions of brownie dough and cookie dough into 1-inch rounds, combining one of each into a single dough ball.

Bake for 8-9 minutes, until edges look set and centers are still soft. Let them cool before enjoying.