Prepare the coffee and let it cool for a while - it should be lukewarm before you use it. Start by separating the egg yolks and whites. Using a whisk, combine the coffee with the oil and lemon zest. Sieve the flour and starches along with the yeast, sugar and vanillin to make the cake mixture even softer and fluffier. After whipping the egg whites into soft peaks, add them to the mixture; remember to fold them in from the bottom up to stop lumps from forming. Grease some single-serve chiffon cake moulds and pour in the mixture. Then bake the cakes for 35 minutes at 150°C (300°F).