Step 1: Prepare the crust
In a mixing bowl, combine the graham cracker crumbs and granulated sugar. Stir in the melted butter until evenly moistened. Press the mixture firmly into the bottom of a 9×13-inch baking dish. Chill in the fridge for at least 15 minutes while preparing the other layers.

Step 2: Make the cream cheese layer
In a large bowl, beat the softened cream cheese with powdered sugar until smooth and fluffy. Fold in the whipped heavy cream gently until fully incorporated. Spread the mixture evenly over the chilled crust and return to the fridge.

Step 3: Prepare the key lime layer
In a clean bowl, whisk together the sweetened condensed milk, key lime juice, and zest. Add the instant vanilla pudding mix and cold milk, whisking until smooth and thickened (about 2–3 minutes). Let it sit for another 5 minutes to firm up slightly.

Step 4: Layer the key lime mixture
Spread the key lime pudding layer evenly over the cream cheese layer. Smooth the top with a spatula.

Step 5: Chill and set
Refrigerate the assembled lush for at least 4 hours, preferably overnight, to allow the layers to fully set and the flavors to meld.

Step 6: Garnish and serve
Before serving, top with whipped cream, extra lime zest, lime slices, or graham cracker crumbs if desired. Slice into squares and serve chilled.