Bloom gelatin sheets (or gelatin powder) in ice cold water. Let them bloom while prepping other ingredients.
Put egg yolks in a large mixing bowl, use a whisk and beat them until thick, approximately 2 minutes. Set up a double boiler (see notes for set up instructions).
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.
Don’t let egg yolks cooked and become scramble .If so, discard the mixture and start over.
Turnoff the heat. Squeeze excess water out of gelatin sheet then put them in the mixture. Stir the mixture until all the gelatin sheets dissolved completely. Take the top of the double boiler off the stove immediately.
Strain the yolk mixture through a fine sieve mesh.
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. Beaton 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 another clean mixing bowl.
Clean the mixing bowl of stand mixer before add heavy cream in. Beat heavy cream on high speed until stiff peaks are formed, about 2 minutes.
Fold ⅓ of egg white mixture into the yolk mixture. When the egg whites combine completely, continue to add another ⅓ 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 3hours.
Serve the cake chill with your favorite garnish.