Preheat the oven to 350F. Line a baking sheet with parchment and set aside.
In a medium mixing bowl, add butter, sugar and brown sugar. Mix Well.
Add milk about 1 tablespoon at a time, mixing well in between additions. This should cream the butter and sugar together well. Stir in the vanilla.
Add the flour, sifted cocoa powder, baking powder, baking soda, sea salt, and cornstarch. Mix until combined to form a cookie dough.
Stir in the mini chocolate chips and white chocolate chips.
Scoop the dough balls out using 1 tablespoon cookie scoop, or your desired size (just remember to adjust baking time according to dough size.)
Place a few extra white chocolate chips on each scoop cookie dough (to make it look pretty, but also extra flavor;)
Bake 12-15 minutes. Cookies that look slightly underdone will result in a more chewy cookie. If you bake them longer, it will result in a crispy cookie. The choice is yours!