First, prepare your curd. In a large microwaveable bowl, whisk together the melted butter, sugar and eggs. Add in the key lime juice and lime zest. Microwave on HIGH power for 1 minute. Stir, then continue microwaving on HIGH power for 1 minute intervals, stirring after each interval, up to 6-7 minutes. At the 6-7 minute mark, the curd should be thick and coat the back of a spoon.
Strain the curd through a fine mesh sieve into a bowl or pitcher - I like using a bowl with a spout for easy pouring. Cool to room temperature for about 1 hour.
While the curd cools, make your crusts. In a large bowl, toss together the graham cracker crumbs, sugar and melted butter until moistened. Portion the crumbs evenly among your glasses and use a Tablespoon measurement to gently compact it. Top with about 1/2 cup of the lime curd - this is for about 4 small parfaits. Adjust accordingly to your choice of glassware.
Refrigerate until cool, about 2 hours. Just before serving, garnish with fresh whipped cream.