Pour milk into a medium saucepan, sprinkle gelatin over the top and let sit for 5 minutes.

Meanwhile, lightly coat six (4- to 5-ounce) bowls or ramekins with spray oil.

Add sugar to saucepan and heat over medium-low heat, stirring very often, until gelatin and sugar are dissolved, 3 to 4 minutes. (The milk should never come to a simmer or boil, so reduce heat as needed.)

Stir in vanilla and salt.

Pour milk mixture evenly into prepared bowls and refrigerate until set, about 8 hours or overnight.

Serve in bowls or invert panna cotta onto small plates.

VARIATIONS: 

-For a chocolate version, before pouring milk mixture into bowls, whisk in 1/3 cup semisweet chocolate chips, melted, and 1 tablespoon unsweetened cocoa powder until well combined. 

-To use agar-agar in lieu of gelatin, stir 1 teaspoon agar-agar powder and 1/4 cup sugar into milk and bring to a simmer over medium heat. Reduce heat to medium-low and simmer, stirring often, until thickened, about 2 minutes. Proceed with the recipe. (This version will set much more quickly.) 

-For extra flavor, infuse milk with a spice such as 3 or 4 star anise, 1 teaspoon crushed cardamom pods or whole cloves, or 3 or 4 cinnamon sticks. Before sprinkling gelatin over milk, heat milk and spice over medium-low heat until very hot, then cover, remove from heat and let sit for 5 to 10 minutes. Strain out spice and cool milk to room temperature, then proceed with the recipe.