In a medium bowl, mix 1 cup of graham cracker crumbs with 4 tablespoons of melted unsalted butter until well combined and resembles wet sand. Reserve ½ cup of the mixture for layering later.

Press the buttered crumb mixture firmly into the bottom of a trifle dish or serving dish, creating an even layer.

In a chilled mixing bowl, beat 1 cup of heavy whipping cream using an electric mixer on medium speed until stiff peaks form, about 3-5 minutes. Set aside.

In another bowl, beat 8 ounces of softened cream cheese until smooth. Add ½ cup of powdered sugar, ½ cup of key lime juice, 2 tablespoons of key lime zest, and 1 teaspoon of vanilla extract. Mix until creamy.

Gently fold the whipped cream into the cream cheese mixture using a spatula until fully combined.

Spread half of the lime filling over the prepared crust, then sprinkle some reserved crumbs on top.

Add the remaining lime filling on top, smoothing it out, then top with the rest of the reserved crumbs.

Cover with plastic wrap and refrigerate for at least 2 hours before serving.