**Lemon Curd:**

1. Fill a saucepan with about 2 inches of water and bring it to a simmer. Place a bowl on top without touching the water.
2. In the bowl, mix lemon juice, eggs, egg yolk, sugar, and salt. Whisk continuously until thick, about 10-12 minutes (or 180°F).
3. Remove from heat and cool to 140°F, stirring occasionally.
4. Cut butter into tablespoon-sized cubes and whisk into the curd one piece at a time until fully melted.
5. Cover the curd with plastic wrap directly touching its surface and refrigerate.

**Lemon Cake:**

1. Preheat the oven to 350°F. Line a 9-inch springform pan with parchment paper and spray with cooking spray.
2. Sift flour, 1/3 cup sugar, baking powder, and salt into a bowl.
3. Add egg, lemon juice, and oil. Mix until smooth and set aside.
4. In a large bowl, beat egg whites until frothy. Add cream of tartar and continue beating until soft peaks form. Add 1/4 cup sugar and beat until stiff peaks form. Fold in lemon zest.
5. Gently fold egg whites into the batter. Pour into the prepared pan.
6. Bake for about 30 minutes or until golden brown and a toothpick comes out clean. Cool completely on a rack.

**Lemon Mousse:**

1. Whip heavy cream in a large bowl until stiff peaks form.
2. Set aside 1/2 cup lemon curd for the cake top. Mix the remaining curd with the whipped cream until well combined.
3. Dissolve gelatin in water in the microwave for about 45 seconds, stirring every 15 seconds.
4. Quickly mix the hot gelatin into the mousse until smooth and evenly combined.

**Assembly:**

1. Run a knife around the cake to loosen it from the pan.
2. Pour the lemon mousse over the cake, spreading it evenly.
3. Chill in the fridge for at least 2-3 hours.
4. Top with the remaining lemon curd and serve.