Genoise Sponge Cake (Torta Genovese)- Liguria, Dolci (Dessert)

Heat the oven to 170 C.
Grease and paper a 23cm cake tin that is 4.5 cm tall.
Whisk together the eggs, yolks, and the sugar in a sauce pan.
Heat the sauce pan over medium heat whisking constantly until the mixture reaches 45u0b0C.
Remove from the heat. Using a handheld mixer or a standing mixer with the whisk attachment, mix together the eggs and the sugar on high continually until the mixture has turned a light tan colour, has at least doubled in volume, has consisten-cy like foam and when you pick up a spoonful and drop it on top it does not sink immediately back into the mixture, about 15 minutes. Add the vanilla and/or lemon zest.
Sift together the flour and the potato starch.
Add two large spoonfuls of the egg mixture to the flours and stir to mix well.
Fold this mixture into the rest of the egg mixture using a silicone or rubber spatula, ensuring that you are scraping the bottom of the bowl each time.
When the flour is well incorporated and there are no clumps, drizzle in the melted butter around the edges of the bowl. Use the spatula to gently incorporate the butter.
Pour into the prepared tin.
Bake for 25-30 minutes or until the top when pressed offers resistance and a skewer insert-ed into the centre comes out clean. Remove from the pan and cool on a wire rack. When cool use a long serrated knife, unflavoured dental floss, or a wire cake cutter to cut the cake in half horizontally to be filled.