Sprinkle the gelatin powder in the ice cold water. Let it bloom while prepping other ingredients.
Meanwhile, separate egg whites and yolks
Put egg yolks in a large mixing bowl, use a whisk and beat them until thick, approximately 2 minutes.
Set up a double boiler. Then cook beaten egg yolks, 1 cup of sugar, lemon zest, lemon juice, and salt over medium heat until sugar dissolved completely and the mixture is thickened, approximately 2 minutes. Remember to stir constantly with a whisk or a rubber spatula.
Turn off the heat. Stir in the mixture until all the gelatin dissolve completely. Remove from the stove.
Strain the yolk mixture through a fine mesh sieve.
Let the mixture cool down at room temperature, about 30 minutes to 1 hour. Stirring occasionally.
Put egg whites in a mixing bowl of a stand mixer fitted with whisk attachment. Beat on medium high speed until foamy. Gradually add the remaining ½ c of sugar to the egg whites. Increase the speed to high and continue to beat until stiff peaks are formed, approximately 2 to 3 minutes.
Transferred egg white mixture to a large, clean mixing bowl.
Beat heavy cream on high speed until stiff peaks are formed. Approximately 1 to 2 minutes
Fold 1/3 of egg white mixture into the yolk mixture. When the egg white combines completely, continue to add another 1/3 of egg white mixture with ½ of whipped cream into the yolk mixture. Continue to fold/whisk until everything combined. Then add the rest of the egg white into this mixture, and continue the process.
Line the side of baking pan with parchment paper. Pour the chiffon mixture onto the crust
Refrigerate the cake until set, at least 3 hours
Serve the cake chill with your favorite garnish.