Prepare the Dough:
Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper or a silicone mat.
In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt. Set aside.
In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy (about 2–3 minutes).
Add the egg, egg yolk, milk, vanilla extract, and red food coloring. Mix until well combined.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.
Fold in the white chocolate chips if using.
Shape the Cookies:
Using a cookie scoop or spoon, scoop dough into balls (about 1.5 tablespoons each) and place them on the prepared baking sheet about 2 inches apart.
Bake the Cookies:
Bake for 10–12 minutes, or until the edges are set and the centers look slightly soft.
Let the cookies cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely.