In the bowl of a stand mixer fitted with the paddle attachment, or in a large mixing bowl using an electric mixer, beat the butter, granulated sugar, and brown sugar until light and fluffy. Make sure that your butter is at room temperature and beat it on a medium speed for about 5 minutes.
Next, add the egg, egg yolk, and vanilla extract to the bowl. Beat on medium speed for another 2-3 minutes, or until creamy.
In a separate bowl, whisk together the flour, cocoa powder, baking soda, espresso powder (if using), and salt.
Add the flour mixture into the creamed butter mixture and beat speed until there are no more dry spots. Once the cookie dough comes together, add the chocolate chips and continue beating on low until they are evenly distributed.
Cover the cookie dough with plastic wrap and chill. For the best tasting cookies, you will want to chill the cookie overnight, but if you are in a rush, 30 minutes - 1 hour will still produce really chewy tasty cookies.
Preheat oven to 350ºF. Line baking sheets with parchment paper
After the cookie dough has had some time to chill, form 3-tablespoon sized dough balls. I like to use a large cookie scoop for this step. Arrange 6 cookie dough balls onto each parchment lined cookie sheet.
Bake the cookies in a fully preheated oven for 10-12 minutes, or until the edges are set and the centers are slightly underbaked. Cool on the pans for 5 minutes before transferring to wire racks to cool completely.
If you want to make these cookies extra decadent, top them caramel filled chocolates after they have cooled for about 10 minutes.