Separate yolks from egg whites. Whisk egg whites and salt until thick. Add 80 g sugar to the mixture. Stir egg yolks and powdered sugar until foamy. Sift some flour over and mix. Carefully knead in egg whites. Spread the sponge cake mixture on a baking tray lined with baking paper until smooth. Bake for approx. 15 minutes in a preheated oven (electric oven: 175° C/ circulating air: 150° C/ gas: see manufacturer’s instructions). Remove the sponge cake plate with the baking paper from the oven, cover with a damp tea towel, and let cool.