Get Ready: Preheat your oven to 350°F (175°C). In a small bowl, whisk together the flour, baking soda, and salt. Set this aside.
Cream the Wet Ingredients: In a large mixing bowl, combine the softened butter, granulated sugar, and light brown sugar. Use an electric mixer to beat them on high speed for 1-2 minutes until the mixture is light and fluffy – this creates a wonderfully tender cookie!
Add Egg and Vanilla: Beat in the room temperature egg and vanilla extract until they are fully incorporated into the mixture.
Combine Dry and Wet: Gently stir in the fresh key lime zest. Then, gradually add the dry flour mixture to the wet ingredients, about a third at a time. Mix just until the dough comes together; be careful not to overmix to keep the cookies soft and chewy.
Shape and Bake: Roll the cookie dough into approximately 1 ½-inch balls and place them onto a baking sheet lined with parchment paper. Bake for about 8-10 minutes. The cookies should look slightly puffy and the edges just barely set.
Cool Down: Let the cookies cool on the baking sheet for 2-3 minutes before transferring them to a wire rack to cool completely. This prevents them from breaking and helps them firm up.
Prepare the Glaze: While the cookies cool, make the refreshing lime glaze. In a bowl, whisk together the powdered sugar, key lime juice, milk (start with 1 tablespoon and add more if needed), vanilla extract, and the ½ teaspoon key lime zest. A pinch of salt is optional but can enhance the flavor! Adjust the milk until you reach a smooth, pourable consistency. Let the glaze sit for about 5-10 minutes to thicken slightly.
Glaze and Serve: Once the cookies are completely cool, dip the tops of each cookie into the key lime glaze, or spread it over with a knife. Allow the glaze to set before serving these delightful summer treats!