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1
Heat the oven to 170 C.
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2
Grease and paper a 23cm cake tin that is 4.5 cm tall.
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3
Whisk together the eggs, yolks, and the sugar in a sauce pan.
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4
Heat the sauce pan over medium heat whisking constantly until the mixture reaches 45u00b0C.
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5
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.
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6
Sift together the flour and the potato starch.
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7
Add two large spoonfuls of the egg mixture to the flours and stir to mix well.
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8
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.
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9
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.
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10
Pour into the prepared tin.
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11
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.
