Place the simple syrup, mascarpone cheese, limoncello, and salt in a food processor. Process until the mixture is smooth. Pour the mixture into an 8-by-8-inch glass baking dish. Freeze for at least 4 hours or until the mixture is firm.
Using the tines of a fork, scrape the mixture into small serving bowls or glasses. Serve immediately.
Lemon Simple Syrup
In a small saucepan, combine the sugar, water, lemon zest, and lemon juice over medium heat. Bring to a boil, reduce heat, and simmer for 5 minutes, stirring occasionally, until the sugar has dissolved. Remove the pan from the heat and allow the syrup to cool, about 20 minutes.