In a large bowl cream together the butter, dark brown sugar, and white granulated sugar until fluffy. Beat in the egg, vanilla and lime zest. Add in the flour, baking powder and salt and mix until just combined. Dough will be very crumbly.
Divide the dough into 6 equal portions and form each portion into a ball. Flatten the balls slightly and place onto a parchment lined baking sheet. Press a cookie scoop, spoon, or finger into the middle of each cookie to create an indent or well. Chill in the refrigerator for at least 30 minutes.
Bake at 350°F for 15-18 minutes until the edges are just turning golden brown, so the cookies to stay chewy. Allow to cool on a wire rack.
While the cookies are baking, make the lime filling by whipping the sweetened condensed milk and the ½ cup of lime juice using an electric mixer or whisk. Chill in the refrigerator.
In a separate bowl, make the buttercream frosting by whipping the butter until fluffy, then adding in the powdered sugar and beat to combine. Whisk in lime juice. Transfer to a piping bag with a large tip (1m is what we prefer).
Pipe a border around the edge of each cookie with the buttercream frosting, then fill the center with your lime filling. Chill in the refrigerator for 15 - 20 minutes, then garnish with sprinkles or lime zest if desired.
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The recipe can be prepared with key limes or regular limes but make sure you are using fresh lime and not bottled lime juice.
If lime filling does not set in the refrigerator, whisk in powdered sugar 1 tablespoon at a time until it is thickened to your liking.
Key Lime Cookies https://www.mamalovesfood.com/key-lime-cookies