Beat the butter in a mixer for 1-2 minutes, until creamy and lighter in color. Add both sugars and beat another few minutes until fully combined and fluffy. Scrape the bowl down well.
Add the vanilla and egg to the butter mixture and beat on high until completely combined.
In a separate bowl, combine the cocoa powder, sugar, baking soda and salt. Add half to the butter mixture, mix on low until just combined, add the rest, and mix again until just combined. Scrape down after combining at each step.
Add the milk and beat on high just until combined. Add ⅔ cup of chocolate chips and mix on low until just combined.
Turn out the dough onto plastic wrap and close tightly. Refrigerate at least 4 hours or overnight.
When ready to bake, preheat the oven to 350°F. Cover two baking sheets with Silpat or parchment paper.
Roll dough into 1 inch balls, then gently press 2-3 chocolate chips into each ball.
If you have time, put the baking sheets in the fridge for another hour. If not, you can skip this step.
Bake cookies for 9-11 minutes. Let sit on the baking sheet for another minute, then transfer to a cooling rack.