BREAD PUDDING: Preheat oven to 325 degrees. Arrange bread cubes in a lightly greased 2 1/2 quart baking dish or a 9 x 9 x 2-inch square baking pan.
In a large bowl, whisk together eggs, sugar, heavy cream, milk, limoncello, vanilla extract and lemon zest; pour over bread. Cover with aluminum foil; let stand 20 minutes.
Bake casserole for 30 minutes; remove foil and bake for 20 minutes or until set and golden.
LIMONCELLO-SYRUP: In a small saucepan over medium heat combine limoncello, lemon zest, lemon juice, and sugar; whisk until sugar dissolves and mixture comes to a boil. Boil 1 minute, remove from heat and allow syrup to cool completely.
Serve pudding warm or cold topped with vanilla ice cream, drizzle with Limoncello Syrup, garnished with fresh basil or mint if desired.