Step 1: Cream the Butter and Sugars

In the bowl of a stand mixer fitted with the paddle attachment, combine the cold butter cubes with the sugars until the butter resembles small pebbles (do not cream together).

Step 2: Add the Wet Ingredients

Beat in the egg, egg yolk, and vanilla until smooth, scraping down the bowl as needed.

Step 3: Mix the Dry Ingredients

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

Step 4: Finish the Dough

Gradually add the dry ingredients to the butter mixture, mixing just until almost combined.

Fold in the chopped semi-sweet and milk chocolate.

The dough should be thick, soft, and slightly sticky—not crumbly. If it feels a little dry, let it rest for 10–15 minutes before making any adjustments. The flour and cocoa continue absorbing moisture as the dough rests.

Step 5: Chill

Cover the bowl and refrigerate for at least 2 hours.

For the thickest cookies and the richest chocolate flavor, chill overnight.

Step 6: Scoop

Preheat the oven to 375°F (190°C).

Line baking sheets with parchment paper.

Using a large cookie scoop (about 3 tablespoons), shape the dough into tall mounds instead of smooth balls. Press a few extra chocolate chunks into the tops for that bakery-style look.

Step 7: Bake

Bake for 10–12 minutes.

The edges should look set while the centers still appear slightly underbaked. Don't worry—they'll finish baking on the hot cookie sheet.

Step 8: Cool

If desired, sprinkle with flaky sea salt immediately after baking.

Allow the cookies to cool on the baking sheet for 10–15 minutes before transferring to a wire rack.