Prep the pan: Line the base of a 9-inch (23 cm) springform pan with parchment. Lightly grease the sides.
Make the crust: Combine crushed biscuits, sugar, and salt. Stir in melted butter until the mixture feels like damp sand. Press firmly into the base of the pan, pushing a little up the sides. Chill for 20 minutes.
Bloom the gelatin: Sprinkle gelatin over 3 tablespoons cold water in a small bowl. Let stand 5 minutes until spongy.
Beat the cream cheese: In a large bowl, beat cream cheese with sugar until smooth and fluffy, 2–3 minutes. Scrape the bowl as you go.
Add the dairy and flavor: Beat in sour cream, vanilla, lime juice, and a pinch of salt until fully combined and silky.
Warm the mango and dissolve gelatin: Gently warm the 1/2 cup mango puree in a small saucepan until just hot (not boiling). Remove from heat. Stir in the bloomed gelatin until completely melted and smooth.
Combine: Beat the warm mango-gelatin mixture into the cream cheese filling on low speed until evenly blended. The filling should look glossy and thick but pourable.
Fill and level: Pour the filling over the chilled crust. Tap the pan gently on the counter to release air bubbles. Smooth the top with an offset spatula.
Chill to set: Cover and refrigerate for at least 6 hours, preferably overnight, until firm.
Make the topping: In a small saucepan, combine passionfruit pulp, mango puree, and sugar. Bring to a gentle simmer over medium-low heat, stirring. Taste and adjust sweetness. If you want it thicker, stir in the cornstarch slurry and simmer 30–60 seconds until glossy.
Cool the topping: Remove from heat and let cool to room temperature. It should pour but not be hot.
Top the cheesecake: Once the cheesecake is fully set, run a knife around the edge and release the springform. Spoon the cooled topping over the surface, letting a few seeds show for texture. Sprinkle with lime zest if using.
Final chill: Refrigerate 30–60 minutes to lightly set the topping before slicing.
Garnish and serve: Add fresh mango slices, extra passionfruit, or mint. Slice with a hot, dry knife for clean edges.