Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper.
To make the chocolate chip cookie dough, cream butter, brown sugar, and granulated sugar until light and fluffy.
Add egg and vanilla, beat to combine.
Stir in flour, baking soda, and salt. Fold in the chocolate chips and set aside.
To make the brownie cookie dough, whisk melted butter and sugar until smooth.
Beat in egg and vanilla.
Then, stir in cocoa powder, flour, baking powder, and salt until just combined. Fold in chocolate chips.
To form the cookies, scoop about 1 tbsp of chocolate chip dough and 1 tbsp of brownie dough. Press them gently together and roll into a ball (you want a half-and-half dough ball).
Place on the baking sheet about 2 inches apart. Bake for 10–12 minutes, until edges are set but centers look slightly underdone (they’ll firm up as they cool).
Cool on the baking sheet for 5 minutes, then transfer to a wire rack.