Add the milk to a medium-sized saucepan. Evenly spread the gelatine powder on top and let it sit for 5 minutes to soften. The soaking or "blooming" of the gelatine is essential. Heat the milk while stirring for the gelatine to completely dissolve. Don't boil it!
1 tbsp gelatine powder, 1/2 cup whole milk
Add the cream and sugar. Heat everything up on medium-heat while stirring to dissolve the sugar. Again, don't boil the mixture as it will impact the gelatin from setting later on (note 1).
2 cups heavy cream, 1/3 cup granulated sugar
Remove the saucepan from the heat and stir in the vanilla extract (note 2). Set aside.
2 tsp vanilla extract
Pour the mixture into 4 dessert glasses or lightly oiled ramekin molds. 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:Make sure the cream mixture needs is between 90°F to 140°F (32°C to 60°C).Note 2: 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.