Drain the peaches and cut into small pieces. First put the frozen raspberries (not defrosted) in a bowl or dessert dish and then spread the peaches on top.
Mix the mascarpone, yogurt and vanilla sugar together. Whip the cream until stiff and carefully fold it into the mascarpone mixture. Spread the whole thing over the fruit and let it steep in the refrigerator for at least three hours, or better overnight.
Sprinkle all over with brown sugar two hours before serving (the cream should no longer be visible). Just before serving, apply a thin, fresh layer of sugar again.