Place the closed ring mould on an oven tray lined with baking paper. Beat the egg whites with the salt until stiff, add half of the sugar and continue to beat until the mixture turns glossy. Add the remaining sugar and vinegar, continue beating until the egg whites are glossy and very stiff. Sieve the cocoa and chocolate powder onto the egg whites and carefully mix. Divide the mixture into the tin, separate from the edge with a spatula, remove the ring mould.
Bake/dry: Preheat the oven to 200 °C then turn down the heat to 150°C. Bake the pavlova for about 20 mins. in the lower half of the oven. Turn down the heat to 120°C, allow the pavlova to dry out for about 1 1/2 hrs., keeping the oven doors ajar with a wooden spoon handle. Remove the pavlova, cool on a cooling rack. Slide the pavlova onto a cake plate.
Beat the cream with the crème fraîche until stiff and spread onto the pavlova. Wash the grapes, toss in the sugar and place on top of the cream.