
Place the water in a bowl and sprinkle the powdered gelatin over it. Gently mix to saturate the gelatin and set it aside to bloom for at least 10 minutes. See recipe notes below on how to bloom gelatin leaves.
In a saucepan, place half of the heavy cream and all of the sugar. Heat over medium heat.
Whisk while heating the cream to dissolve the sugar. When the cream mixture is steaming, or just starting to simmer, remove the saucepan from the heat.
Add the bloomed gelatin and whisk it in to dissolve it in the hot mixture. Make sure there are no lumps or undissolved granules.
If needed, you can heat the mixture on low heat while whisking to dissolve anything that is not dissolving properly, but do not let the mixture boil. Stir in the vanilla and salt.
Transfer the mixture into a jug or container with a spout to make it easier to pour it out later.
Add the remaining heavy cream and stir to mix. Next, stir in the buttermilk.
Allow the mixture to cool down to room temperature completely. Whisk this mixture vigorously to ensure that the gelatin, buttermilk, and cream mix homogeneously. You can also use a stick blender to make sure it mixes well.
Take 6 metal molds that are about 5 - 6 fl oz capacity (155 - 180 mL). Lightly grease the insides with butter or oil spray. Use a paper towel to remove excess grease, since you don't want the oil to mix with the panna cotta. You can also use 6 ramekins (or similar), to serve the panna cotta directly in the dishes.
Portion 120 - 130 mL of the mix into each mold / ramekin (about ½ cup).
Cover loosely with plastic wrap and refrigerate for at least 8 hours.
Unmold the set panna cotta onto individual serving dishes. See all the tips described in the post on how to unmold panna cotta easily.
Or serve the panna cotta directly in the set dishes (without unmolding).
Top the panna cotta with some fruit coulis or syrup or fresh fruits (like mango that I use here), or any other toppings of your choice.