CRUST: Line an 8 or 9-inch square pan with parchment paper (butter the pan first so it sticks). Crush the graham crackers and place in a bowl. Mix in the confectioner's sugar and salt. Add the melted butter and mix until the crumbs clump when squeezed. Press firmly and evenly into the pan. Freeze at least 3 hours.

LEMON CURD: Bloom 1 tsp gelatin in 2 tbsp water; set aside. In a non-reactive saucepan, combine the sugar, zest, cornstarch, and 2 tbsp water; whisk in a little lemon juice until smooth, then add the rest. In a separate bowl, whisk the egg yolks with 2 tbsp water. Heat the lemon-sugar mixture over medium-high, stirring, until it boils; boil 2 minutes. Temper the yolks with spoonfuls of the hot mixture, then return the yolks to the pan. Cook over medium-high, stirring, until it simmers and thickens (about 5 minutes). Off heat, whisk in the butter, then stir in the bloomed gelatin until dissolved. Cool to room temperature, whisking occasionally to prevent a skin (or press plastic wrap onto the surface).

CHEESECAKE LAYER: Bloom 2 tsp gelatin in 3 tbsp water, at least 10 minutes. Beat the cream cheese and sugar with a paddle until smooth. Add the lemon juice, zest, vanilla, and salt; mix until smooth. Separately, whip the chilled heavy cream to soft peaks and fold into the cream cheese mixture. Microwave the bloomed gelatin 10–20 seconds until dissolved (not boiling); temper with 1 tbsp cream. With the mixer on high (whisk attachment), add the tempered gelatin near the whisk; mix about 1 minute until combined, then whisk smooth by hand.

ASSEMBLY: Spread the cheesecake layer over the frozen crust evenly. Whisk the cooled (not hot) lemon curd until smooth and pour over the surface; spread evenly. Swirl the curd into the top of the cheesecake with a knife, keeping swirls near the surface. Cover and refrigerate at least 6 hours, preferably overnight.

Lift out by the parchment and cut into squares with a clean, warm, sharp knife, wiping between cuts.