Add the gelatine powder to the cold water. Stir and let it sit for 5 minutes to soften. The soaking or "blooming" of the gelatin is essential. Make sure it's completely dissolved.
7 g gelatine powder, 3 tbsp water
Combine the cream, milk, and sugar in a medium saucepan. Bring everything to a simmer while stirring to dissolve the sugar. Don't boil the mixture as it will impact the gelatin from setting later on. Remove the saucepan from the heat and stir in the vanilla extract (note 1). Set aside.
2 cups heavy cream, 1/2 cup whole milk, 1/3 cup granulated sugar, 2 tsp vanilla extract
Make sure the cream mixture needs is between 90°F to 140°F (32°C to 60°C). Add the dissolved gelatine to the mixture while whisking vigorously for 2-3 minutes.
Strain the mixture through a fine sieve carefully into 4 dessert glasses or ramekin molds to strain off any gelatin lumps. Place the cups in the fridge for a minimum of 4 hours or overnight to set.
Rhubarb Coulis
Add the rhubarb pieces to a medium saucepan alongside the sugar, water, and salt.
2 cups rhubarb stalks, 1/3 cup granulated sugar, 1/8 tsp salt, 1/2 cup water
Cook everything on low medium-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 the rhubarb coulis when serving after the panna cotta has fully set in the fridge. Serve the panna cotta cups cool or at room temperature.
Notes
Note 1: If you're using a vanilla bean, scrape the seeds into the cream mixture at the start and let the whole bean simmer in the mixture. Remove the bean after step 2, squeezing it out throughly.Storage: You can store it in the fridge for 2 days which makes it a great make-ahead dish.