Preheat oven to 350°F. Place 12 jumbo-size (3 1⁄2-inch) aluminum foil baking cups in a regular-size muffinpan, and coat with cooking spray.

Stir together graham cracker crumbs, coconut, melted butter, and 4 tablespoons of the sugar in a bowl until combined; firmly press onto bottom and up sides of each baking cup (about 3 rounded tablespoons of mixture per cup). Bake in preheated oven until lightly browned, about 5 minutes. Set aside. Keep oven set at 350°F.

Whisk together sweetened condensed milk, Key lime juice, egg yolks, salt, and 1 tablespoon of the lime zest in a bowl until combined. Pour mixture evenly into prepared crusts (about 1⁄4 cup per crust). Bake until pies are just set, 12 to 14 minutes. Cool completely on a wire rack, about 1 hour. Refrigerate 2 hours, and then remove aluminum foil baking cups before serving.

Beat heavy cream with an electric mixer on medium-high speed until foamy. Gradually add vanilla extract and remaining 1 tablespoon sugar; beat until medium peaks form. Fold in remaining 1⁄2 teaspoon lime zest. Top each mini pie with a dollop of whipped cream, and garnish with a Key lime slice.