1. Preheat the oven to 350°F.

2. Using an electric mixer, beat the egg whites until stiff peaks form. Set aside.

3. In a bowl, beat the sugar and egg yolks until the mixture is frothy. Add the lemon zest and mix. Add the flour, salt, and baking powder. Add the melted butter.

4. Gently fold in the beaten egg whites.

5. Sugar a cake mold. Fill the mold with the mixture.

6. Bake for 25 minutes.

7. In a saucepan, heat the lemon juice and an equivalent amount of sugar. Reduce by half over low heat.

8. Remove the cake from the oven. Pour the syrup over the cake and serve in large portions.