Beat the yolks with the water and sugar until they are fluffy (about 10 minutes with a hand mixer).
Sift together the flour, starch and baking powder and stir into the yolk mixture. Beat the egg whites with a pinch of salt and fold in (best with a whisk or wooden spoon, then the snow in the dough stays nice and “fluffy”).
Spread the dough on a baking tray lined with baking paper and bake at 200 °C for about 10-15 minutes (middle shelf).
After baking, turn the sponge out onto a clean tea towel and roll up loosely to cool (leave the baking paper on top).
Wash, hull and quarter the strawberries. Set aside a few pretty strawberries for garnish. Whip the whipped cream with the cream stiffener, powdered sugar and vanilla sugar until stiff.
Carefully roll out the cooled sponge cake and peel off the baking paper. Spread the sponge with 2/3 of the whipped cream and sprinkle the strawberry quarters on top.
Roll up and decorate the sponge roll with the remaining cream and the remaining strawberries.