Combine half of the cream and the milk in a medium saucepan. Bring everything to a simmer while stirring to dissolve the sugar. Don't boil the mixture. Remove the saucepan from the heat and set aside.
2 cups full-fat cream (35%), 1 cup milk, 1/3 cup sugar, 1 tsp vanilla extract
In the meantime, add the gelatine powder to the cold water. Stir and let it sit for 5 minutes to soften (bloom).
3 tsp gelatine powder, 2 tbsp water
Whip the other half of the cream and vanilla extract until stiff peaks form.
Add the dissolved gelatine to the warm cream mixture while stirring. Cool it down in the fridge or freezer for 10-15 min until it starts to thicken.
Strain and pour it into 4 dessert glasses or ramekin molds and let it cool to room temperature. Place it in the fridge for a minimum of 4 hours or overnight until set.
Rhubarb Coulis
Add the stalks to a medium saucepan alongside the sugar, water, and salt.
2 cups rhubarb stalks, 1/3 cup sugar, 1/8 tsp salt, 1/2 cup water
Cook everything on low heat for 10-15 minutes until the rhubarb starts to fall apart and the liquid has a syrup-like consistency.
Let the rhubarb cool down and purée it. Add it to the panna cotta and serve cool.
Notes
Don't boil the cream mixture as it will impact the gelatin from setting.Soaking or "blooming" of the gelatin is essential. Make sure it's completely dissolved.You have to cool down the cream and gelatin mixture before adding the whipped cream or the whipped cream will melt.Add the rhubarb coulis when serving after the panna cotta has fully set in the fridge.