Halve the grapes. Heat butter in a non stick pan, add grapes and toss. Add brown sugar and toss and let the sugar melts.
Divide them equally into 4 ramekin moulds. Place dollops of mascarpone cheese over the grapes and spread lightly.
Sprinkle brown sugar over and place the moulds in a salamander. Cook at 200°C till the cheese softens and sugar begins to melt. Serve hot.