Start by grating the outer part of a lemon using a fine grater or zester, setting the zest aside for the final decoration on the tiramisu.
In a medium-sized pot, combine 1 liter of water with 300g of sugar. Heat the pot over medium-high heat, until the sugar completely dissolves into the water.
Once the sugar has dissolved, introduce the juice of 3 lemons into the pot, using a strainer to prevent any seeds from get into the syrup.
With a potato peeler, carefully remove the outer peel of a lemon, ensuring to avoid the bitter white part, and add it to the water.
Allow the water, infused with sugar, lemon juice, and peel, to boil for a minimum of 5 minutes.
After 5 minutes, switch off the heat and incorporate 100ml of Limoncello into the syrup, gently stirring it in.
Next, strain the lemon-infused syrup into a bowl to filter out any lemon peels. Let the syrup cool at room temperature for about an hour before using it.