Cream the butter and sugars together with a hand or stand mixer fitted with a paddle attachment on medium speed until light and fluffy. Scrape down the sides and bottom as needed. Beat in the egg and vanilla on high speed until combined, scraping down the sides as needed. In a separate bowl, combine the flour, cocoa powder, baking soda, and salt. Slowly beat it into the wet ingredients on low speed. The dough will be thick. Again on low speed, beat in the milk and then the white chocolate chips. The cookie dough will be sticky and tacky.

Cream the butter and sugars together with a hand or stand mixer fitted with a paddle attachment on medium speed until light and fluffy. Scrape down the sides and bottom as needed. Beat in the egg and vanilla on high speed until combined, scraping down the sides as needed. In a separate bowl, combine the flour, cornstarch, baking soda, and salt. Slowly mix it into the wet ingredients on low speed and then beat in the chocolate chips. The dough will be thick.

Cover both cookie doughs tightly and chill in the refrigerator for at least 3-4 hours or up to 3 days. It’s imperative that both doughs are chilled otherwise the cookies won’t hold shape.

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

Measure 1.5 Tablespoons of chocolate cookie dough (I like using this medium cookie scoop). Roll into a ball. Measure 1.5 Tablespoons of chocolate chip cookie dough. Roll into a ball. Press the two balls together between your hands and roll them into 1 large ball. Repeat for each cookie. The chocolate cookie dough is particularly sticky and will stick to your hands, so wash/wipe them after you roll each.

Bake the cookies for 12-13 minutes or until the edges appear set and the centers still look soft. Tip: If they aren’t really spreading by minute 9, remove them from the oven and lightly bang the baking sheet on the counter 2-3x. This helps initiate that spread. Return to the oven to continue baking.

Cool cookies for 5 minutes on the baking sheet. During this time, I like to press a few more white chocolate chips + semi-sweet chocolate chips into the tops of the warm cookies. (This is optional and only for looks.) Transfer to cooling rack to cool completely. The cookies will slightly deflate as they cool.

Cover leftover cookies tightly and store at room temperature for up to 1 week.