Add the brown sugar and butter to a medium sized mixing bowl, then use a hand or stand mixer to beat the mixture together until it's light and fluffy (~2-3 min.)
Add in the egg and vanilla and beat until well combined (~1 min.)
Add in the flour, baking powder, salt, and coffee grounds. Use a rubber spatula to fold the ingredients together just until no flour streaks remain.
Cover the bowl then place in the fridge to chill for at least 30 minutes.*
Once the cookies have chilled, preheat the oven to 350°F / 175°C. Line a cookie sheet with parchment paper or with a silicone baking mat.
Use a 3 tbsp. cookie scoop to dish out the cookies onto the prepared cookie sheet, spacing them apart to allow room for spreading.
Bake for 10-12 minutes, until the sides of the cookies just start to turn golden brown, and the top of the cookies looks just slightly wet. Immediately after taking the cookies out of the oven, take a bowl that is slightly larger than the cookies and swirl it around over the cookies to make them a little more uniform and circular.
Let the cookies rest on the hot baking sheet for at least 5 minutes before transferring them to a wire rack to cool.