In a small bowl, sprinkle the gelatin over the cold water and let it sit for about 5 minutes to bloom.

In a medium saucepan, combine the heavy cream, whole milk, sugar, vanilla extract, and lemon zest. Heat over medium heat, stirring occasionally, until the mixture is hot but not boiling.

Remove the saucepan from heat and stir in the bloomed gelatin until fully dissolved. Add the fresh lemon juice and mix well.

Pour the mixture into individual serving cups or ramekins. Allow to cool to room temperature, then cover with plastic wrap and refrigerate for at least 4 hours or until set.

To serve, run a knife around the edge of the panna cotta and invert onto a plate, or serve directly in the cups. Garnish with additional lemon zest or fresh berries if desired.