First, juice the grapefruits and strain the pulp from the liquid with a fine mesh strainer. Pour 1 cup (240ml) of the grapefruit juice into a small sauce pot and add in the sugar. Dissolve the sugar over low heat, to make a grapefruit syrup. Stir or swirl every minute or so.
When done, remove the syrup from heat and stir in the rest of the grapefruit juice and the Campari. Chill the sorbet base thoroughly, then freeze in an ice cream maker according to the manufacturer’s directions.
The machine should finish when the sorbet is at a soft serve consistency. Scrape the sorbet out of the ice cream maker into a lidded container. Immediately place the soft sorbet into the freezer for at least another 4 hours or overnight for best results.
When completely frozen, find an ice cream or cookie scoop that matches the opening of your glass in diameter, so they fit. Scoop sorbet spheres into your glass and top with sparkling wine. Serve with a spoon much like you would a rootbeer float.