In a medium bowl, whisk together the all-purpose flour, cocoa powder, baking soda, and salt. Set aside.

In a large mixing bowl, using an electric mixer, cream together the softened unsalted butter, granulated sugar, and light brown sugar on medium speed until light and fluffy (about 2-3 minutes).

Beat in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract until just combined.

Gradually add the dry ingredient mixture to the wet ingredients, mixing on low speed until just combined and no streaks of flour remain. Do not overmix.

Fold in both the semi-sweet chocolate chips and milk chocolate chips using a rubber spatula until evenly distributed throughout the dough.

Cover the bowl with plastic wrap and chill the cookie dough in the refrigerator for at least 30 minutes. This helps prevent the cookies from spreading too much and deepens the flavor.

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

Using a 1.5-inch cookie scoop (or about 1.5 tablespoons), drop rounded balls of dough onto the prepared baking sheets, spacing them about 2 inches apart.

Bake for 10-12 minutes, or until the edges are set and the centers still look slightly soft. Do not overbake for chewy cookies.

Remove from the oven and let the cookies cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.