Pour the milk into a medium saucepan. Sprinkle the gelatin onto the milk in an even layer and set aside for 5 minutes for the gelatin to “bloom.” The surface of the milk will turn dry and wrinkly.

Turn the heat to low and cook, stirring often, until the gelatin is dissolved, about 2 minutes. Add the sugar, turn off the heat and stir until the sugar is dissolved, returning the pan to low heat to rewarm if needed. Stir in the vanilla and salt.

Pour the milk mixture through a sieve into a large bowl other heat-proof container with a pouring spout. Add the cream and sweet potato puree and stir to combine. Divide the mixture among the prepared glasses or ramekins, cover with plastic wrap, and refrigerate until set, at least 4 hours.

Serve the panna cottas directly from the ramekins  
Top the panna cottas with marshmallow cream and toast slightly and serve.