In a saucepan, combine heavy cream, whole milk, and sugar. Heat over medium heat until the sugar dissolves and the mixture is hot, but not boiling.

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

Remove the cream mixture from heat and stir in the bloomed gelatin and vanilla extract until fully dissolved.

Pour the mixture into ramekins and refrigerate for at least 4 hours or until set.

In a saucepan, combine chopped rhubarb, sugar, and water. Bring to a boil over medium heat.

Reduce heat and simmer for about 10-15 minutes until rhubarb is soft. Stir in lemon juice.

Remove from heat and let cool. Strain if desired for a smoother sauce.

Once the panna cotta is set, remove from the refrigerator and top with rhubarb sauce before serving.