Lay out muffin liners or line a cookie sheet with waxed paper. Cut marshmallows in half if desired or just leave them whole. Place rice krispies in a medium bowl.
Place the sugar, cream, and corn syrup in a small saucepan over medium heat, and stir constantly until the sugar dissolves. Stir in the butter until melted. Cook without stirring until firm ball stage (248 degrees F), then remove from the heat immediately.
Stir in the vanilla and salt. Allow the caramel to cool to approximately 175 degrees F.
Using a fork, drop a marshmallow half in the caramel and turn it over until it is completely covered. Remove the marshmallow from the caramel and drop it in the cereal, rolling until it is covered. Place in a muffin liner or on a cookie sheet lined with waxed paper to set.
Repeat the caramel and coating steps with the remaining marshmallows. If the caramel becomes too stiff for easy dipping, place it over the heat for a minute or two until it becomes easy to work with.
Allow the candies to set fully at room temperature before serving. Store excess candies in an airtight container at room temperature or in the fridge for longer storage (but bring to room temperature before serving).