For the blueberry roulade, preheat the oven to 200°C (top and bottom heat). Line a baking tray with baking paper.
Separate eggs, beat egg whites with sugar and salt until firm. Gradually stir in yolks. Sift together flour, cornflour, cocoa and baking powder and fold into the egg mixture until you have an even batter.
Spread evenly on the baking sheet and bake for about 10 minutes. In the meantime, have a clean dish towel ready and sprinkle with a little sugar.
Turn the cake upside down onto the tea towel, loosen the baking paper and roll it up with the help of the tea towel. Allow to cool well.
Whip the cream with cream stiffener until firm. Spread on the cooled, rolled out roulade, sprinkle with blueberries and roll up again. Sprinkle the blueberry roulade with powdered sugar and enjoy.