Cream butter, turbinado sugar, and brown sugar until smooth.
Mix in egg, sourdough starter, vanilla, key lime juice, and lime zest.
Add flour, baking soda, and salt. Mix until a dough forms.
Scoop about 75g of dough, roll into balls, and lightly flatten.
Roll in the sugar and lime zest coating.
Chill dough for about 1–2 hours.
Bake at 350°F for 15–18 minutes until edges are set and centers are soft.
Cool on the pan for 10 minutes before moving to a rack.
