In a large mixing bowl, beat 1 cup of room temperature unsalted European-style butter with 1/2 cup of granulated sugar and 1/2 cup of light brown sugar for 1-2 minutes until pale and fluffy. Add 1 large egg and 1 yolk, along with 1 teaspoon of vanilla extract, mixing for another 3 minutes until creamy and well combined.

In a separate bowl, whisk together 1 teaspoon of cornstarch, 1/2 teaspoon of baking powder, 1/4 teaspoon of baking soda, 1/2 teaspoon of kosher salt, and 1/2 cup of Dutch process cocoa powder. Gradually add this dry mixture to the wet ingredients, mixing until fully incorporated.

Now, slowly add 1 3/4 cups of all-purpose flour to the dough on low-medium speed. Once combined, gently fold in 1 1/2 cups of bittersweet chocolate wafers and 4 to 5 oz of finely chopped milk chocolate using a spatula.

Scoop the cookie dough into 6 oz mounds and place them on a parchment-lined tray, leaving space between each. Chill the dough uncovered in the refrigerator for at least 4 hours or overnight.

Preheat your oven to 350°F (175°C). Arrange the chilled dough mounds on a baking sheet and bake in the preheated oven for 18-20 minutes.

Allow the cookies to rest on the baking sheet for 3 minutes before transferring them to a cooling rack to cool completely.