In a small saucepan, combine the orange juice, lemon juice, sugar and salt. Warm over medium heat to just below a simmer, stirring occasionally with a wooden spoon to help dissolve the sugar. Once the sugar is dissolved, remove the saucepan from the heat and stir in the liqueur. Set aside to cool completely.
Pour into an 8-inch baking dish and freeze until completely frozen, at least 4 hours and up to overnight.
Scrape the surface of the frozen mixture with a fork to form large shards of ice. Spoon about 1/2 cup into coupe glasses and top with prosecco.