In a mixing bowl toss the sliced peaches and brown sugar. Pour the prosecco and Grand Marnier on top and gently press down to make sure all of the peaches are covered.

Cover with cling film and refrigerate for at least 4 hours up to 12 hours.

In the bowl of a stand mixer whisk the heavy cream on high speed until soft peaks form. Add the vanilla, mascarpone, and powdered sugar. Continue to whisk until stiff peaks form.

To serve spoon the peaches with a small amount of the prosecco into a dessert dish and top with a dallop of whipped mascarpone. Pour an additional splash of Grand Marnier if desired.