Put the Prosecco, 1/2 cup of the sugar and 1/2 cup water in a large saucepan and bring to a boil over high heat. Reduce the heat to a simmer and cook for 5 minutes to burn off some of the alcohol. Remove from the heat and slowly whisk in the gelatin until completely dissolved. Pour into a 9 by 11-inch baking dish, cool to room temperature, then refrigerate to set up for at least 4 hours.
In a large bowl, whip the cream to medium stiff peaks. In another bowl, whisk the remaining 1/4 cup sugar into the mascarpone until smooth. Add 1/3 of the whipped cream to the mascarpone to lighten, and then fold in the remaining whipped cream. Cut the Prosecco gelatin into 1 by 1-inch cubes and put a layer into the bottoms of 4 parfait glasses. Divide 1/2 of the cream mixture among the glasses on top of the gelatin, and then repeat with 2 more layers, ending with a final dollop of the cream. Serve with a long spoon and dig in!