In a small saucepan set over medium heat, brown the butter. Stir the butter occasionally until it turns amber in color and develops little brown specks.
Transfer the brown butter to a medium sized mixing bowl and set it in the freezer for 5 minutes to chill. You should be left with at least 90g of brown butter, if you have a little less (2-3g), just add in some water until you reach 90g. If you have way less than 90g, you cooked the butter for too long and will need to start again.
Once the butter has cooled down a bit, whisk in both sugars.
Whisk in the egg and vanilla until well combined.
Add in the flour, baking powder, baking soda, salt, and coffee grounds. Use a rubber spatula to fold the mixture together just until a few flour streaks remain.
Add in the flour, baking powder, baking soda, salt, and coffee grounds. Fold the mixture together with a rubber spatula until almost all of the flour has been incorporated.
Add the chocolate into the flour mixture and fold the mixture together just until no flour streaks remain. Cover the bowl tightly with plastic wrap and place it in the fridge for 30 minutes to firm up.**
Once you're ready to make the cookies, preheat the oven to 350°F / 175°C. Line a cookie sheet with parchment paper.
Scoop out heaping 3 tbsp. scoops of the cookie dough, spacing them out on the cookie sheet to allow room for spreading.*** (If you chilled the dough for longer, you may need to wait 10-20 minutes for it to become soft enough to scoop.)
Top the cookies with more chocolate (optional), then bake for 10-12 minutes, or until the edges of the cookies start to turn golden brown.
Once the cookies come out of the oven, I recommend "scooting" them. This just means take a bowl, cup, or biscuit cutter that is slightly larger than the cookie, place it on top of the cookie, and swirl the bowl around to make the cookie a little more round.
Let the cookies sit on the hot baking tray for at least 5 minutes before transferring them to a wire rack to cool. Sprinkle with flaky sea salt, serve and enjoy!