- 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