Add limoncello, gin, lemon juice, simple syrup, and egg white (if using) into a shaker with no ice. Shake for 15 seconds to foam.
Add ice and shake again until cold.
Strain into a chilled champagne glass.
Top slowly with prosecco or sparkling wine.
Gently stir and garnish with a lemon twist or mint.
Notes
Use fresh lemon juice for the brightest flavor. Adjust sweetness with simple syrup as needed. Skip gin for a lighter drink. Serve in a chilled glass to keep bubbles lively.