Whisk together flour, baking soda, and espresso powder in a medium bowl and set aside.

In a saucepan over medium-high heat, melt 1 stick of butter (8 tablespoons). Continue cooking until butter begins to brown. Swirl pan constantly until butter has foamed and takes on a dark golden brown color and smells nutty. Watch carefully, as it can quickly go from dark to burnt. Carefully pour browned butter into the bowl of a stand mixer, scraping all the browned bits from the pan by using a heat-proof spatula.

Immediately whisk the remaining butter pieces into the browned butter, until melted. Add sugars, salt, and vanilla to the bowl, and whisk to fully incorporate.

Add egg and egg yolk and whisk until smooth with no lumps remaining, about 30 seconds. Let mixture stand for 2-3 minutes to let the sugars melt, then whisk for another 30 seconds. Repeat this rest-and-whisk process 3 times, total. Mixture should seem thickened, smooth, and shiny.

Using a spatula, stir in dry ingredients until just combined (and a few flour streaks still remain). Stir in chocolate chips until evenly distributed and no flour pockets remain. Refrigerate dough for at least 2 hours, or overnight.

Once dough has chilled, scoop into balls that are slightly bigger than a golf ball. If the dough was in the refrigerator overnight, let it rest at room temperature for 10-15 minutes before scooping. (The dough balls may be frozen at this point).

Place cookies 2” apart on baking sheets lined with parchment paper or a SilPat.

Bake in a pre-heated 375°F for 10-14 mins** until cookies are golden brown and puffy in center, but edges are darker and have set. Let the cookies rest on the baking sheet for at least 5 minutes before transferring to a rack to cool completely.