In a medium saucepan, combine the heavy cream with the milk and ½ cup of the sugar. Heat over medium heat until warmed and bubbles form around the edges. Remove from the heat, stir in the prepared popcorn, and cover. Let the mixture steep for 30 minutes.
Return the pan back to the burner over medium heat. In a medium bowl, combine the egg yolks and the remaining ¼ cup of sugar. Whisk until smooth. When the mixture in the pan is warmed through again, slowly pour into the bowl with the egg yolks, whisking constantly. Return the entire mixture to the saucepan and place over medium heat. Cook, stirring constantly and scraping the bottom of the pan, until the mixture thickens slightly and coats the back of a spoon (170˚-175˚ F on an instant read thermometer.) Pour the custard through the mesh strainer into a storage container. Stir in the vanilla and salt. Let cool slightly, then cover and refrigerate until completely chilled.
Freeze in an ice cream maker according to the manufacturer’s instructions. Store in an airtight container in the freezer. To make sundaes, layer with a handful of caramel corn and a drizzle of caramel sauce.