Pour 1 cup of the heavy cream into a saucepan and sprinkle the gelatin over it. Let it sit and bloom for 10 minutes.
If using an ice water bath, fill a large bowl or sink with ice water while the gelatin is blooming.
After 10 minutes, heat the cream and gelatin mixture on medium high heat and stir constantly until the gelatin is dissolved, about 2-3 minutes (about 135 ℉ on an instant read thermometer if being precise)
Turn down the heat and add the sugar. Stir to dissolve.
Slowly pour the remaining cream and limoncello into the saucepan, while stirring.
If using a ice water bath, transfer the panna cotta to a bowl and set it in the prepared ice water bath.
Slowly hand whisk or stir the mixture until it starts to slightly thicken and lowers in temperate, about 5-10 minutes. It should be around 50-55 degrees if using a thermometer.
Strain the mixture to remove any lumps, then divide evenly into desired serving dishes or tarts. See notes if using ramekins.
Cover each serving container with plastic wrap. You can put all the containers on a baking pan or sheet so they’re easy to move around and then cover the entire sheet with plastic wrap.
Let chill in the fridge at least 4 hours or overnight.
Top with more limoncello or berry coulis before serving.