First, zest and juice at least 8 key limes, so you have at least 4 tablespoons of juice and 2 teaspoons of zest. Weigh or measure the sugar and set it aside. Now, weigh or measure the flour, baking powder, and salt in a bowl. Run a whisk through to blend.
Cream (baking term) the butter and cream cheese using a mixer. Use a medium speed for about a minute. Next, scrape down the bowl sides and add the sugar. Mix on medium-high speed for 2 to 3 minutes. Next, add the vanilla extract, key lime juice, and key lime zest, mixing until incorporated.
Next, add one egg at a time. Scrape down the sides of the bowl and across the bottom and mix for another 30 seconds to ensure the eggs are incorporated. Finally, turn the mixer on to a slow speed and add the flour mixture. This is a soft cookie dough, so to firm up the butter, I put the cookie dough into the refrigerator for 30 minutes.
Preheat the oven to 350°F (175°C or 180°C) and line 2 cookie sheet pans with parchment paper. Using a medium cookie scoop, add 12 cookie dough mounds to a prepared sheet pan. Bake for 10-12 minutes. Once out of the oven move the cookies to a cooling rack. Make sure the cookies are cooled before making the frosting.