Preheat your oven to 350 degrees F. Coat the cups of a mini muffin tin with non-stick cooking spray. Set aside.
Divide the prepared cookie dough into 24 one-teaspoon balls. Place one cookie dough ball into each muffin tin cup.
Bake for 10-13 minutes until the cookies are just starting to brown.
Remove from the oven and use the handle of a wooden spoon to gently press down the center of the cookies to form a cup. Allow to cool in the pan for 5 to 10 minutes before transferring to a wire rack to cool completely.
Key Lime Pie Filling
Whisk the egg yolks, lime juice, sweetened condensed milk, and vanilla together in a medium saucepan over medium-low heat. Cook for 10-15 minutes until the lime filling has thickened and is heated through.
Remove from the heat and allow to cool completely, about 1 hour. (I make the filling while the cookies are cooling so they finish together.)
Pour the filling into a pastry bag and fill each cookie cup to the top. Place the cookie cups into the refrigerator until chilled, about 1 hour.
Top each cookie cup with a dollop of whipped cream and half a lime slice for serving.
Notes
I usually make a full batch of cookie dough. Then I portion the 24 cookie cups and bake off the rest of the dough as regular cookies following the original recipe. They always go fast!
In a pinch, I've used lime juice and it worked just fine.