Browning the butter is technically optional but the extra time is SO worth it! The biggest problem I see people run into when using brown butter in cookies, is not letting the butter cool completely to room temperature before adding it to the dough. So I highly recommend browning the butter the night before you want to make these espresso cookies.
Using a stand or hand mixer, cream together the brown butter, brown sugar, and granulated sugar until smooth and creamy.
Then add in the espresso powder and cream into the butter and sugar for 1-2 minutes until fully combined.
Add in the eggs and vanilla extract and beat until the texture is light and fluffy (about 3-5 minutes, it should look much paler than before).
Stir in the flour, baking soda, and salt and mix until thoroughly combined. The dough should be wet and sticky.
Using a spoon or spatula, gently fold in the dark chocolate wafers.
Spoon a generous 3 tablespoons of dough and roll into balls. If desired, press extra chocolate wafers into the tops of the dough balls.
Cover the dough and chill in the fridge overnight (minimum of 2 hours if you just can't wait).
Preheat your oven to 350°F and line a light colored baking sheet with parchment paper.
Place dough balls at least 2-3 inches apart on the baking sheet (they will spread, I usually only do 6 cookies on a half sheet pan).
Bake cookies for 9-11 minute or until the edges are set and the centers are still soft. Remove from the oven and let rest on the baking sheet for 1-2 minutes, then transfer to a cooling rack.
Sprinkle with flaky sea salt and enjoy!
Notes
To make brown butter, melt unsalted butter over medium heat, stirring frequently. Watch closely for the butter to develop brown specs and a nutty aroma. Once you start seeing brown specs, let cook for just a few seconds longer (watch very carefully or it will burn). Remove from the heat and pour into a small bowl so that it doesn't continue to cook. Set aside and let cool for about 20 minutes in the fridge until soft and pliable. Note: For every ½ cup (1 stick or 113g), you'll lose about 1 tablespoon (14g) to evaporation while cooking. So you can either add in an extra 1 tablespoon before browning, or if you're browning big batches, just make up the weight difference in your recipe with regular unsalted butter if needed.Store cookies in an airtight container at room temperature for up to 4 days (they'll definitely be gone before then though).Freeze pre-rolled cookie dough balls in the freezer for up to 6 months.