1. In a small bowl, sprinkle gelatin over cold water and let it sit for 5 minutes to bloom.
2. In a saucepan, combine heavy cream, whole milk, and sugar. Heat over medium heat until the sugar is dissolved and the mixture is hot but not boiling.
3. Remove from heat and stir in the bloomed gelatin until fully dissolved.
4. Add lemon zest, vanilla extract, and chopped basil leaves. Let it steep for 10 minutes.
5. Strain the mixture through a fine sieve into a mixing bowl to remove the basil leaves and zest.
6. Pour the mixture into ramekins and refrigerate for at least 4 hours or until set.
7. To serve, run a knife around the edge of each panna cotta and invert onto a plate.