Preheat the oven to 200 °C top and bottom heat.
Separate the eggs. Beat the egg whites with the lemon juice and salt until semi-stiff. Add half of the sugar and continue beating until stiff.
Mix the flour with the cornstarch. Beat the egg yolks with the remaining sugar and 1 tablespoon warm water until foamy. Sift the flour mixture over it. Add the beaten egg whites and fold everything in. Pour the fluffy dough onto a baking sheet lined with baking paper, smooth it out and bake in a hot oven on the middle rack for 10-12 minutes.
Turn the sponge out onto a cloth sprinkled with sugar and peel off the baking paper. Cover with a damp kitchen towel to cool.
For the filling, mix the gelatin with the powdered sugar. Add the cream and whip until stiff. Spread two-thirds of the whipped cream on the sponge cake and top with three-quarters of the raspberries. Roll up compactly and place on a plate.
Spread the remaining whipped cream on top and garnish with the remaining raspberries and elderflowers.
Preparation Tip:
You can also prepare the recipe with other berries.