Preheat oven to 375 °F.
In a large bowl, beat the room temperature butter and brown sugar together until light and fluffy. (Around 2 minutes) Add milk, vanilla, and egg. Beat until combined.
In a medium bowl, whisk together flour, cocoa powder, salt, and baking soda. Add dry ingredients to the wet ingredients. Mix until combined. Stir in the white chocolate and semi sweet chocolate chips (if using).
Using a 2 inch cookie scoop, scoop rounded balls of dough onto an ungreased cookie sheet (you can also use a spoon). Keep the cookies about 2 inches apart.
Bake for 8-10 minutes for chewy cookies. Cool completely on a wire rack.