Preheat your oven to 350°F (175°C) and prepare a baking sheet lined with parchment paper.

In a medium bowl, combine the all-purpose flour, unsweetened cocoa powder, baking soda, and salt. Whisk together until blended.

In a large mixing bowl, cream together the softened unsalted butter, granulated sugar, and brown sugar until light and fluffy.

Add the egg and vanilla extract to the mixture and beat until fully combined.

Gradually mix in the dry ingredient mixture until combined, avoiding overmixing.

Fold in the semi-sweet and milk chocolate chips gently using a spatula.

Scoop and shape the dough onto the prepared baking sheet, leaving space between each cookie.

Bake for 10-12 minutes, until the edges are set and the centers look slightly underbaked.

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