Add melted butter to crumbs, and sprinkle into the bottom of your pans or serving bowls. Tamp down with the bottom of a glass to form a crust.
In a separate bowl, mix condensed milk and lime juice together.
In a small saucepan, sprinkle gelatine over ¼ c. cream. Allow it to rest 1-2 minutes, then stir it well over low heat until the gelatine dissolves completely.
Add sugar to the remaining cream, and beat until fluffy while you allow the saucepan mixture to cool.
Slowly add the mixture from your saucepan to the whipped cream, mixing on low. Then, fold the lime mixture in as well until fully blended.
Spoon onto crust(s) and refrigerate for 2-3 hours. Top with whipped cream minutes prior to serving, and optionally, garnish with lime zest and/or lime slices.
Recipe by Happy Hour Projects at https://happyhourprojects.com/key-lime-mousse/