Heat oven to 350°F. Lightly butter a 9-inch pie plate.
Stir crumbs, 2.5 tbsp sugar, and melted butter until evenly moistened.
Press mixture firmly into plate and up the sides. Bake 8–10 minutes; cool completely.
Bloom gelatin by sprinkling it over cold water; let stand 5 minutes.
Whisk yolks, 0.8 cup sugar, lemon juice, zest, and salt in a saucepan.
Cook over medium, stirring constantly, until thickened and 160°F, about 5–7 minutes.
Remove from heat and stir in bloomed gelatin until fully dissolved.
Cool lemon base to just warm, stirring occasionally to prevent a skin.
Beat egg whites with cream of tartar to glossy medium peaks.
Fold whites into lemon base in three additions until no streaks remain.
Pour filling into cooled crust. Chill until set, at least 4 hours.
Whip cream with vanilla to soft peaks. Spread or pipe on pie; garnish with zest.