Whisk together the flour, baking powder, and salt and set aside. Cream the butter and sugar and lime zest in a mixing bowl and beat until smooth. Add the egg, vanilla, lime juice and key lime zest. Stir to combine. Finally, add the dry ingredients and mix until combined.
2 cups All-purpose flour, 2 teaspoons Baking Powder, ¼ teaspoon Kosher Salt, ½ cup Unsalted Butter, 1 cup Sugar, 2 tablespoons Key lime zest, 1 Egg, 1 teaspoon Vanilla extract or vanilla paste, 1 tablespoon Lime juice