In a large bowl or the bowl of a stand mixer fitted with the paddle attachment (or using an electric hand mixer), cream the room-temperature butter, granulated sugar, and light brown sugar on medium speed for 2 to 3 minutes, until light and fluffy. Scrape down the sides and bottom of the bowl with a rubber spatula.

Add the egg and beat on medium speed for 20 seconds. Add the egg yolk and repeat. Scrape down the sides and bottom of the bowl. Add the vanilla extract and milk, and mix on low until fully combined.

Add the flour, dark cocoa powder, baking soda, cornstarch, kosher salt, and the 6 ounces of chopped chocolate to the wet ingredients. Mix on low speed just until a few streaks of flour remain. Don’t overmix.

Cover the bowl and refrigerate the dough for at least 30 minutes. This keeps the cookies thick and helps them hold their shape in the oven.

Preheat the oven to 350°F and line two baking sheets with parchment paper.

Scoop the dough into 3-tablespoon balls and place them about 2 inches apart on the prepared baking sheets, about 6 per tray.

Break the 2-ounce topping bar into big chunks and press one chunk onto the top of each dough ball.

Bake for 15 to 16 minutes, until the edges are set but the centers still look slightly soft. They’ll continue to set as they cool.

Let the cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.