In a medium bowl, mix together the graham cracker crumbs, granulated sugar, and melted butter until it resembles wet sand. Firmly press this mixture into mini tart pans and chill in refrigerator for at least 15 minutes.

In a separate bowl, combine sweetened condensed milk, freshly squeezed key lime juice, and lime zest. Whisk until smooth. Pour the filling into the chilled crusts and smooth the tops.

Beat the heavy cream and powdered sugar until stiff peaks form, about 3 to 5 minutes on medium-high speed.

Using a piping bag or spoon, add whipped cream on top of each pie, and sprinkle additional lime zest for garnish.

Serve chilled, garnished with lime slices or toasted coconut if desired.