Place 4 coupe or flute glasses in the freezer for at least 15 minutes before you plan to serve.
Scoop the frozen lemon sorbet into a chilled 2-quart / 1.9 L mixing bowl.
Pour in the vodka and whisk briefly with a hand mixer or balloon whisk until the sorbet just starts to break down at the edges, about 20 seconds.
Pour in the prosecco gradually while whisking constantly, working quickly, until the mixture turns pale and frothy and falls from the whisk in ribbons, about 30 to 45 seconds. Stop as soon as it thickens like a milkshake, longer whisking turns it watery.
Pour immediately into the chilled glasses, filling each about three-quarters full.
Garnish with a curl of lemon zest and a small mint leaf if using.
Serve right away with a small spoon or straw, since sgroppino melts within 5 to 10 minutes.