How to Make It
Roll pie dough to a 1/8-inch thick circle and place inside a greased 9-inch metal tart pan or pie pan. Cut excess dough, leaving a 1/2-inch overhang. Fold overhang under itself, creating a thicker ring of crust around the pie’s edge. Crimp edge. Take a sheet of aluminum foil and grease it liberally with butter. Place the buttered surface inside the pie shell, allowing the pie dough to come in direct contact with the aluminum foil. Freeze crust for 1 hour or overnight. Preheat oven to 350°F. Bake crust with aluminum foil for 20 minutes. Remove aluminum foil and use a spoon to press down any areas that have puffed up. Return to oven for an additional 13-15 minutes, until the crust is golden brown all over. Set aside to cool.
Place gelatin in 3 Tbsp. water and stir to combine; set aside. Zest all 3 lemons into a small bowl; set aside. Juice each lemon until you have 1/3 cup juice with no seeds; set aside. Over a double boiler, combine 1/2 cup sugar, 2/3 cup water, egg yolks, and lemon juice. Cook, stirring occasionally, until thickened, about 25-30 minutes. Whisk in gelatin mixture until completely dissolved. Add half of the lemon zest and refrigerate for 30 minutes, whisking every 5 minutes, until consistency has slightly thickened and mixture has cooled completely.
Beat egg whites on medium-high until soft peaks form. Slowly stream in remaining 1/4 cup sugar and beat until stiff peaks form. Fold egg white mixture into chilled lemon mixture in 3 additions, waiting until each addition is incorporated before adding the next. Pour filling into pie crust and refrigerate until set, about 2 hours or overnight.
Whip cream with powdered sugar until stiff peaks form. Add half of remaining lemon zest and mix to combine. Scoop whipped cream onto the center of the pie and spread it slightly around, leaving much of the lemon chiffon surface exposed. Sprinkle the entire pie with remaining lemon zest and serve.