Preheat oven to 350 degrees F. Line 2 baking sheets with silicone liners or spray lightly with cooking spray. Set aside.

In the bowl of a stand mixer, beat together the butter and ONE (1) cup of the powdered sugar until creamy, about 2 minutes. Add in the key lime juice, the zest and vanilla and beat to combine. Lastly, add in the flour and cornstarch until a thick dough has formed.

Roll Tablespoon-sized balls of dough and place them 1" apart from one another onto the prepared baking sheets. Gently press them down with a sugared glass.

Bake for approx. 9-11 minutes, rotating pans halfway through baking time, until golden brown and set. Cool completely on the baking sheets. Remove to a wire rack and one by one, gently coat in the remaining powdered sugar. Serve!