Line a 9-inch (23 cm) square metal pan with parchment paper, leaving an overhang on two opposite sides. Lightly grease the exposed sides so the filling releases cleanly.

Combine the graham cracker crumbs, granulated sugar, and salt in a medium bowl. Pour in the melted butter and stir until every crumb is evenly moistened. The mixture should resemble damp sand and hold together when pressed.

Transfer the crumb mixture to the prepared pan: Press it firmly and evenly across the bottom using the flat base of a measuring cup. Pay particular attention to the corners, where loose crumbs can cause the bars to break.

Pour the cool water into a small microwave-safe bowl. Sprinkle the gelatin evenly over the surface rather than dropping it in one pile. Let it stand for 5 minutes. The mixture should become thick and wrinkled as the gelatin absorbs the water.

Place the softened cream cheese in a large mixing bowl. Beat on medium speed for 1 to 2 minutes, until completely smooth. Scrape the bottom and sides of the bowl to remove any firm lumps.

Gradually beat in the sweetened condensed milk on low speed. Continue mixing until the filling looks smooth, glossy, and uniform.

Mix in the lemon juice, lemon zest, vanilla, and salt on low speed. The filling will begin to thicken slightly as the lemon juice combines with the condensed milk.

Microwave the bloomed gelatin for 8 to 12 seconds, just until it becomes clear and liquid. Do not allow it to boil. Stir well and let it cool for about 1 minute; it should remain fluid.

Whisk approximately 1/4 cup (60 ml) of the lemon filling into the dissolved gelatin. Slowly pour this mixture back into the main bowl while beating on low speed. Mix just until evenly incorporated. This gradual method reduces the risk of gelatin strands or lumps.

In a clean medium bowl, whip the cold heavy cream on medium-high speed until soft peaks form. The cream should hold a curved peak when the beaters are lifted and still look smooth rather than stiff.

Fold one-third of the whipped cream into the lemon mixture to lighten it. Gently fold in the remaining whipped cream in two additions. Stop when no white streaks remain. The finished filling should be thick, airy, and spreadable.

Spoon the lemon filling over the chilled crust: Spread it evenly into the corners with an offset spatula, then smooth the top. Tap the pan gently on the counter once or twice to release large air pockets.

Cover the pan without touching the filling and refrigerate for at least 6 hours, preferably overnight. The center should feel firm and should not ripple when the pan is moved.

Use the parchment overhang to lift the chilled dessert from the pan. Sprinkle with optional lemon zest and powdered sugar shortly before serving.

Use a large, sharp knife to cut the dessert into 16 squares. Wipe the blade clean with a warm, damp towel between cuts for smooth sides and distinct layers.