In a large bowl, beat 1 cup unsalted butter, 1¼ cups brown sugar, and ½ cup granulated sugar together for 3 to 4 minutes until pale and fluffy.
Add 2 large eggs and 2 large egg yolks, one at a time, mixing well after each addition. Stir in 1 tablespoon vanilla extract until fully combined. The dough should look smooth and glossy by the time everything is combined.
In a separate bowl, whisk together 3¼ cups all purpose flour, ¾ cup unsweetened cocoa powder, 3 tablespoons cornstarch, 1 teaspoon baking soda, 1 teaspoon baking powder, 1 teaspoon salt, and ½ teaspoon espresso powder until evenly combined.
Add the dry ingredients to the wet ingredients and mix just until combined. Do not overmix. Overmixing is one of the fastest ways to make cookies tougher.
Fold in 1½ cups chopped dark chocolate chunks until evenly distributed throughout the dough.
Cover the dough and chill for at least 1 hour. Chilling ensures the cookies stay thick and develop deeper flavor.
Preheat the oven to 375°F. Line baking sheets with parchment paper.
Using a ¼ to ½ cup cookie scoop, portion the dough into tall mounds and place at least 3 inches apart on the prepared baking sheets. Do not flatten the dough.
Bake for 12 to 14 minutes, or until the edges are set and the centers still look slightly soft.
Remove from the oven and sprinkle lightly with flaky sea salt, if using. Let the cookies cool on the baking sheet for 10 minutes before transferring to a wire rack. The centers will continue to set as they cool.