- Coat four 5-ounce ramekins or 6-ounce custard cups lightly with flavorless oil.
- Pour 1/4 cup of the cream into a small heatproof bowl, sprinkle the gelatin over it and let stand until softened, about 10 minutes. Place the bowl in a larger bowl of hot water and stir until the gelatin has dissolved.
- Meanwhile, in a saucepan over medium heat, bring the remaining 1 1/4 cups cream, the mascarpone, sugar and salt just to a boil. Remove the pan from the heat, add the chocolate and whisk until smooth.
- Add the gelatin mixture to the chocolate mixture and stir until well blended. Pour through a fine strainer into a 4-cup glass measuring cup or a bowl with a pour spout. Divide the mixture evenly among the ramekins and let cool to room temperature.
- Cover the panna cotta loosely and refrigerate until set and thoroughly chilled, at least 3 hours or up to 1 day.
- To serve, dip the ramekins one at a time into a bowl of hot water for about 5 seconds, then run a table knife around the edges of the custard and invert onto a chilled serving plate.