Get your key lime syrup ready. Dissolve granulated sugar in a mixture of key lime juice and water in a pot, then let cool completely.
Whip up the creamy filling. Beat mascarpone cheese, sweetened condensed milk, lime juice, heavy cream, and salt until smooth and creamy.
Assemble and serve. Layer ladyfingers followed by the filling and repeat, finishing with the filling on top. Refrigerate up to overnight, grate some lime zest over the top, slice, and serve.
