Preheat the oven to 340°F (170°C). Use a pastry brush to grease the inside of a tube pan with softened butter. Sprinkle it with granulated sugar, and rotate the mold so that the sugar sticks to the butter all over the surface. Refrigerate the mold until ready to use.

To make the cake batter, sift the flour and potato starch mixture in a large bowl and set aside. Use an egg separator to separate the whites from the yolks.

In a separate bowl, whisk the egg yolks with 1 cup (125 g) of icing sugar until whitish and doubled in size.

Sift the dry ingredients over the egg yolk mixture, add the lemon zest, and gently mix with a rubber spatula.

In a clean bowl, whip the egg whites until mounted, and gradually add the remaining 1/3 cup (45 g) icing sugar and lemon juice. Beat the meringue until stiff and glossy. Egg whites must be whipped until the consistency of a bird's beak (bec d’oiseau in French).

Add 1/3 of the beaten egg whites into the principal preparation and gently mix. Add the rest of the meringue and carefully mix by lifting and turning the bowl in the same direction.

Pour the batter into the prepared mold to fill up to 2/3. Tap the mold on the work surface, and bake for 40 minutes or until a toothpick inserted in the center comes out clean.

Remove the cake from the oven and let it rest in the mold for a few minutes. Unmold and let it cool to room temperature. To serve, sprinkle the cake with icing sugar.