Sprinkle gelatin powder over 3 tablespoons cold water in a small bowl and let sit 5-10 minutes until gelatin swells and looks spongy.
2.5 teaspoons gelatin powder, 3 tablespoons cold water
Combine heavy cream, whole milk, sugar, and scraped vanilla bean pod and seeds in a saucepan over medium heat. Stir occasionally until sugar dissolves and mixture reaches 170-180°F, removing just before it boils.
2 cups heavy cream, 1 cup whole milk, 0.33 cup granulated sugar, 1 vanilla bean vanilla bean
Remove saucepan from heat. Remove vanilla pod, add bloomed gelatin, and whisk gently for 1-2 minutes until gelatin dissolves completely with no visible granules.
2.5 teaspoons gelatin powder
Pour mixture through a fine mesh strainer into a measuring cup. Then divide evenly among four lightly greased 4-6 oz ramekins or molds.
Cover each ramekin with plastic wrap and refrigerate at least 4 hours or overnight until panna cotta feels firm but yields slightly when pressed gently with fingertip.