Mix the graham cracker crumbs with the butter until thoroughly blended. I like to do this with my fingers. Set aside.
Whip the heavy cream. I do this right in my food processor. Process until the cream is thick and holds its shape. This won't take long, don't over whip.
Add the cream cheese, breaking it up into chunks as you add it, and process until smooth. Process in the sweetened condensed milk and lime juice until smooth and creamy.
Put 2 tablespoons of the crumbs at the bottom of each of 6 parfait jars or glasses. Top with about 1/4 cup of the key lime mixture. Add 2 more tablespoons of crumbs, topped with another 1/4 cup of lime mixture.
Cover and chill the parfaits until ready to serve. I recommend at least 2 hours.
Whip the cream with confectioner's sugar to top the parfaits before serving. Garnish with lime zest.
I like to make this whole recipe using my food processor. I first use it to crush my graham crackers crumbs. Then I whip the cream by processing briefly until it thickens, then add the remaining ingredients per the recipe. It's quick and mess free, and I can use cold cream cheese straight from the fridge. If you don't have a food processor you can do this with electric beaters, but it will be more difficult to get the cream cheese to incorporate, you'll have to make sure it's softened first.