Prep your crust: If using a pastry crust, blind-bake it and let it cool completely. For a graham crust, press crumbs into a pie dish and bake for 8–10 minutes at 350°F (175°C), then cool.

Bloom the gelatin: Sprinkle gelatin over 1/4 cup cold water in a small bowl. Let it sit for 5 minutes until it looks spongy and absorbed.

Make the lemon base: In a saucepan, whisk egg yolks, 3/4 cup sugar, lemon juice, lemon zest, milk (or half-and-half), and a pinch of salt. Cook over medium-low heat, stirring constantly, until the mixture thickens slightly and coats the back of a spoon, 5–7 minutes. Do not let it boil.

Add the gelatin: Remove the pan from heat and stir in the bloomed gelatin until fully dissolved. Add vanilla if using. Transfer to a large bowl to cool to room temperature, about 20–30 minutes, stirring occasionally.

Whip the egg whites: In a clean bowl, beat egg whites until soft peaks form. Gradually add 2 tablespoons sugar and beat to glossy medium-firm peaks.

Fold it together: Once the lemon base is no longer warm to the touch (cool but not set), gently fold in the whipped egg whites in three additions. Use a light hand to keep the mixture airy.

Fill the crust: Pour the chiffon filling into the cooled crust. Smooth the top with a spatula.

Chill to set: Refrigerate for at least 4 hours, preferably overnight, until the filling is fully set and sliceable.

Top and serve: If desired, whip 1 cup heavy cream with a tablespoon of sugar to soft peaks and spread or pipe over the chilled pie. Garnish with extra lemon zest or thin lemon slices.