Line baking sheets with baking parchment or waxed paper.
Grease waxed paper lightly.
Draw twelve 5-inch (13-cm) circles on baking parchment or waxed paper with a pencil.
Preheat oven to 425°F(220°C).
To make cake, beat egg whites until stiff; fold in half the sugar.
Beat 5 egg yolks and remaining sugar in a medium bowl set over hot water until thick and creamy.
Remove from heat.
Sift flour, baking powder and salt together; fold into egg yolk mixture with butter.
Fold in beaten egg whites.
Put batter into a pastry bag fitted with a plain nozzle.
Pipe batter onto circles in a spiral pattern; smooth the surfaces.
Bake sponges 10 to 12 minutes or until just firm to the touch.
Invert sponge rounds onto a cloth towel with lining paper still attached.
Sprinkle lining paper with cold water and peel it off gently.
Fold sponges carefully in half.
Cool under cloth towel.
To make filling, beat cream until stiff.
Put whipped cream into a pastry bag fitted with a fluted nozzle.
Place a little fruit in each cooled sponge shell; fill with piped whipped cream.
Sprinkle shells with powdered sugar.