Heat oven to 325°F. Wrap a 9-inch springform pan in foil. Set a kettle of water to boil.
Stir crumbs, 2 tbsp sugar, and a pinch of salt. Mix in melted butter until evenly moistened.
Press crumbs into the pan base. Bake 8–10 minutes until fragrant. Cool while making filling.
Beat cream cheese, sugar, and 1/4 tsp salt until silky, scraping the bowl often.
Blend in cornstarch. Mix in sour cream and heavy cream until smooth but not airy.
Stir in lemon zest and vanilla. Add lemon juice last and mix just to combine.
Pour filling over crust. Place pan in a large roasting pan and add hot water halfway up the sides.
Bake 55–65 minutes until edges are set and center has a slight wobble.
Turn off oven, crack door, and rest cheesecake 45–60 minutes. Cool to room temperature, then chill 4 hours.
Whisk sugar, cornstarch, and a pinch of salt in a saucepan. Slowly whisk in water and lemon juice.
Cook over medium heat, stirring, until thick, glossy, and bubbling, 3–5 minutes. Remove from heat.
Stir in butter and lemon zest. Cool 10 minutes, then spread the lemon layer over the chilled cheesecake. Chill 20 minutes.
Beat meringue powder, warm water, and cream of tartar on medium until foamy. Increase speed to medium-high.
Gradually add sugar and whip to stiff, glossy peaks. Beat in vanilla briefly.
Pile meringue onto cheesecake and swirl. Torch lightly or broil 1–2 minutes until golden. Chill 30 minutes before slicing.