Using a hand-held or stand mixer, beat the butter, granulated sugar, and dark brown sugar until fluffy and light in color, about 2-3 mins on med/high speed.
Add the egg and vanilla and beat until well combined.
In a medium-sized bowl, whisk together the flour, unsweetened cocoa powder, dark cocoa powder, baking soda, and salt until well combined.
With the mixer on low speed, gradually add the flour mixture to the butter mixture until well combined (the dough will be thick)
With the mixer on med/high speed, add the milk and chocolate chips (the dough will be a bit sticky).
Cover the bowl with plastic wrap and refrigerate the dough for 3 hrs to overnight. Do not skip this step.
Before baking, let the dough sit at room temperature for 15 mins. Preheat the oven to 350 F and line two baking sheets with parchment paper or a silicone mat.
Using a medium cooking scoop (about 1.5 tablespoons), scoop the dough and roll into balls (or cylinders if you want a thicker cookie). Wash your hands every so often to remove any dough stuck to them.
Arrange the balls 2-3 inches apart on the prepared baking sheets, sprinkle coarse sea salt over them, and bake one baking sheet a time, for 12 minutes.
Cool on the baking sheet for about 5-8 minutes before transferring them to a cooling rack. Store them in an airtight container for up to a week at room temperature.