Preheat the oven to 350°F / 175°C. Line a cookie sheet with parchment paper or with a silicone baking mat.
To a medium sized mixing bowl, add in the brown sugar and key lime zest. Use your fingers to rub the zest into the sugar (or you can beat the mixture with a hand mixer.)
Add in the butter and beat the mixture until it's light and fluffy (2-3 min.)
Beat in the egg until well combined.
Add in the flour, baking powder, and salt. Mix the ingredients together just until no flour streaks remain.
Cover the dough and leave it in the fridge to chill for at least 30 minutes, or for up to 2 days.*
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. Evenly space the cookies apart to allow room for spreading. Bake for 10-12 minutes, until the sides of the cookies start to turn golden brown.
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.