In a large bowl with a handheld or stand mixer beat the butter and sugars until smooth.
Add in egg, milk and vanilla and continue to combine. Scrape down the sides and bottom of the bowl as needed.
Gradually add the flour, cocoa powder, baking soda and salt. The dough will be quite thick and sticky.
Add the white chocolate chunks.
Cover dough and refrigerate for at least a couple hours, preferably overnight.
When ready to bake preheat your oven to 350 degrees.
Prepare a cookie sheet lightly sprayed with cooking spray.
Make discs about ¾ of an inch high and 1 ½ inches wide.
Place on the cookie sheets and bake for 11-14 minutes.
These cookies will look undercooked, especially in the center. They will have started to set around the edges.
Allow to cool for at least 5 minutes on the cookie sheet before placing them on wire racks. Enjoy!