Start by zesting enough key limes to get roughly 2 tablespoons of zest and set aside. Cut key limes in half and juice enough until you have a ¼ cup of juice and set that aside as well.

In a mixing bowl, add 2 ¼ cups all-purpose flour, ¾ teaspoon baking soda, ¾ teaspoon salt, and the 2 tablespoons of key lime zest. Whisk until fully combined and set the dry ingredients aside.

In the bowl of a stand mixer, or a large mixing bowl if using a hand mixer, add ¾ cup softened butter, ¾ cup dark brown sugar, and ½ cup white sugar. Mix on medium speed until the butter and sugar are creamed, about 3-4 minutes. Add the ¼ cup key lime juice and ½ teaspoon vanilla extract and give it a quick mix, now add the egg and egg yolk, one at a time, mixing just enough until you no longer see streaks of egg.

Begin adding the dry ingredients into the wet ingredients, about ⅓ of the flour mixture at a time. Mix at low speed and add more of the dry ingredients as it combines. Mix until just combined and avoid overmixing. Now add 1 ¼ cups of white chocolate chips and stir with a spatula to fold them in. Refrigerate the cookie dough for about 1 hour.

Preheat the oven to 325°F / 160°C and line a half-sheet baking sheet with parchment paper or a silicone liner. Take the cookie dough out of the fridge and let it soften slightly while your oven preheats.

Using a 1.5 tablespoon cookie scoop, or whatever you use to make cookies, scoop the dough and add rounded balls to the baking sheet, leaving about 2 inches between them.

Bake the key lime cookies for 10-12 minutes and remove them from the oven. They should look slightly undercooked with just barely a bit of browning on the outside. Leave them on the cookie sheet for about 5 minutes so they can finish baking and setting, then transfer to a cooling rack.

Continue this process until all the cookie dough is used, keeping the dough refrigerated while a batch is baking. While the cookies are cooling on a rack, add fresh key lime zest over the top of the cookies. This adds lovely color and some added flavor to the cookies. Serve warm or allow to fully cool before storing.