Beat the sugar, egg yolks and water using the whisk on a hand mixer until the mixture is light and fluffy. Beat the egg whites with the salt until stiff. Gradually add the sugar and continue beating until the egg white turns glossy. Sieve the flour, add to the mixture in layers along with the egg whites and carefully fold in using a rubber spatula. Spread the sponge mixture into a rectangle (approx. 1 cm thick) on a baking tray lined with baking paper.
Approx. 6 mins. in the centre of an oven preheated to 220°C. Remove the sponge, tip the cake and paper out onto a fresh sheet of baking paper, wipe the upper layer of paper with a damp towel and peel away carefully. Cover the sponge immediately with the upturned tray, leave to cool.
Beat the cream with the sugar until stiff. Set aside ¼ of the mixture.
Spread the whipped cream over the sponge, leaving a border of approx. 2 cm all round. Scatter the grapes and chocolate on top of the whipped cream. Roll up tightly from the narrow end. Cover the roulade with the reserved whipped cream, top with the reserved grapes and chocolate.