This cookie dough requires at least 3 hours of chilling, but I prefer to chill the dough overnight. The colder the dough, the thicker the cookies.

In a large bowl using a hand-held or stand mixer fitted with a paddle attachment, beat the butter, granulated sugar, and brown sugar together on medium high speed until fluffy and light in color, about 2-3 minutes.

Add the egg and vanilla extract, and then beat on high speed until combined. Scrape down the sides and bottom of the bowl as needed.

In a separate bowl, whisk the flour, cocoa powder, baking soda and salt together until combined.

With the mixer running on low speed, slowly pour into the wet ingredients. Beat on low until combined. The cookie dough will be quite thick.

Switch to high speed and beat in the milk, then the white chocolate chips. The cookie dough will be sticky and tacky.

Cover dough tightly and chill in the refrigerator for at least 3 hours and up to 3 days. Chilling is mandatory for this sticky cookie dough.

Remove cookie dough from the refrigerator and allow to sit at room temperature for 10 minutes. If the cookie dough chilled longer than 3 hours, let it sit at room temperature for about 20 minutes.

Preheat oven to 350°F (177°C). Line large baking sheets with parchment paper or silicone baking mats. Set aside.

Scoop and roll dough, a heaping 1.5 Tablespoons of dough each, into balls. Arrange 2-3 inches apart on the baking sheets.

Bake the cookies for 11-12 minutes or until the edges appear set and the centers still look soft.

Cool cookies for 5 minutes on the baking sheet. Transfer to cooling rack to cool completely.