Line an 18 cm springform pan with baking paper, preheat the oven to 175 degrees.
Dissolve the coffee powder in 25 ml of hot water, allow to cool. Melt the butter and let it cool down too.
Separate the egg, beat the egg white with half of the sugar until stiff. Mix egg yolks, remaining sugar and vanilla sugar on the highest setting until creamy until light yellow. First stir in the coffee, then the butter. Mix the flour and baking powder, also briefly stir in at the highest level. Finally, carefully fold in the egg whites.
Bake at 175 degrees for about 35 minutes. (But I have to say that I bake in the gas oven, i.e. only with bottom heat. With top / bottom heat, it`s best to do a chopstick test after 20-25 minutes.)
After cooling, dust with powdered sugar or cover with dark couverture as desired and let the cake steep for another 2 - 3 hours.
Tip:
This small, fluffy cake also tastes good as a chocolate version, instead of coffee, simply mix 12 g baking cocoa with the flour.
Double the amount of dough baked in the small pan is enough for 3 - 4 people.