First, add all of the key lime filling ingredients to a medium bowl and whisk until combined. Set aside in the fridge to chill and thicken up while you prepare the cookies.
½ cup + 2 tbsp full-fat plain greek yogurt, ⅓ cup sweetened condensed milk, 2 tbsp fresh key lime juice, 1 tbsp key lime zest, ½ tsp vanilla extract, pinch of kosher salt
Next, preheat your oven to 350F and line an extra-large baking sheet (21x15") or 2 normal size baking sheets with parchment paper. In a medium bowl, whisk together the graham cracker crumbs, flour, salt, baking powder & soda until combined. Set aside. 1 cup gluten free graham cracker crumbs, 1 ¼ cup gluten free 1-to-1 flour, ½ tsp kosher salt, ½ tsp baking powder, ¼ tsp baking soda
In a large bowl (or the bowl of a stand mixer), add cane sugar, brown sugar and key lime zest. Use your hands to massage the key lime zest into the sugar - this releases the oils in the zest and adds extra flavor! Then, add in butter and beat with a hand or stand mixer for 2 minutes, or until fluffy.
½ cup unsalted butter, ¼ cup cane sugar, ¼ cup brown sugar, 1 tbsp key lime zest
Add in egg, key lime juice and vanilla extract and beat for another minute. It won't look fully combined - that's okay! Finally, add in the dry ingredients and switch to a spatula to stir until combined.
1 large egg, 1 tbsp fresh key lime juice, 1 tsp vanilla extract
Measure out 1.5 tbsp (or 1.5oz) of dough, roll into balls, and place at least 2" apart on the prepared baking sheet(s). Use the back of a rounded tablespoon (or your thumbs) to create a deep well in the center of each cookie. The dough is a bit sticky, so it's helpful to spray the back of the tablespoon with some cooking oil.
Transfer cookies to the oven to bake for about 12 minutes. The cookies will have puffed back up, so immediately after removing them from the oven use the back of a rounded tablespoon to create a well in each. Allow them to rest on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.
Once the cookies are completely cool, add about 1 tbsp of the key lime filling to each well. Spread it out with a small knife or offset spatula. Garnish each with a key lime slice and more key lime zest. Best enjoyed chilled, but can be eaten right away too!
key lime slices, key lime zest