Grease the side of the cake pan with a slight amount of softened butter, then line with parchment paper. Return it to the refrigerator for later use.
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 setup 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 overcook and become scramble. If so, discard the mixture and start over.
Turn off the heat. Squeeze excess water out of the gelatin sheet and then put it in the mixture. Stir the mixture until all the gelatin sheets dissolve 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. Stir occasionally.
Put heavy cream in a mixing bowl of a stand mixer fitted with whisk attachment. Whip the heavy cream using a clean whip until stiff peaks form on medium speed. Set aside for later use.
Clean a mixing bowl and whisk attachment if necessary.
Whip the egg whites using a clean whip & bowl until soft peaks form (on medium speed). Then add the remaining sugar. Continue whipping until medium peaks are formed at medium-high speed.
Immediately after the egg whites reach their desired peak, gently fold them into the cooked egg yolks.
Fold ⅓ of the whipped egg whites into the yolk mixture (make sure it is at room temperature) to lighten it to a thicker consistency.
Then gently but thoroughly fold in the remaining whipped egg whites.
Then, fold the whipped cream into the cake batter. The more quickly the two egg mixture and whipped cream are incorporated, the less the cake's volume is lost.
Immediately after mixing, pour the batter into a lined cake pan and put it in the fridge to set.
Refrigerate the cake until set, at least 3hours.
Serve the cake chill with your favorite garnish.