Fill a large bowl or pan with a generous amount of ice and cold water to create an ice bath. Place a large, clean metal bowl in the ice bath and fit the bowl with a fine-mesh sieve. Set aside.
In a large, heavy-bottomed saucepan over medium heat, combine 2 cups heavy cream, 1 cup Lucky Charms cereal milk (from recipe above), and 2 teaspoons kosher salt and cook, stirring occasionally, until hot and jusssttt about to boil, but not boiling.
Meanwhile, spilt the vanilla bean length wise and use the tip of your knife to scrape out the insides into a medium bowl. Don't throw out the pod though, because you'll need that for later! Add 1 cup granulated sugar and 3 tablespoons malt powder and use your fingers to rub all of it together to incorporate and evenly distribute the vanilla seeds throughout the sugar. Whisk in 3 large egg yolks and 1 teaspoon pure vanilla extract until a uniform color, and return to your cream and milk mixture from the previous step.
Remove the cream mixture from heat, and slowly, slowly pour about half of the hot cream mixture into the yolk mixture, whisking constantly. Transfer the yolk mixture back to the saucepan with the remaining cream mixture and return it to medium heat. Cook, stirring constantly with a heatproof rubber spatula, occasionally scraping the bottom of the saucepan to prevent any scorching, until the liquid begins to steam, about 2 to 3 minutes.
Remove the custard from heat and IMMEDIATELY pour it through the sieve and into the bowl in the ice bath. Tuck the vanilla bean pod back into the custard. Allow the mixture to cool completely, stirring every 5 minutes or so to release heat.
When the custard has completely cooled, cover the bowl and transfer to the refrigerator to steep and chill for at least 1 hour, but preferably overnight. When you are ready to freeze the custard, fish out the vanilla bean pod, and transfer the custard to an ice cream maker and spin according to the manufacturer's instructions. When the ice cream has finished churning and has the consistency of soft serve, transfer to a medium bowl. Use a rubber spatula to stir in 1 1/2 cups to 2 cups of Lucky Charms marshmallows until they are fully incorporated throughout the ice cream. Eat immediately, or transfer to an airtight container to freeze and eat later at your leisure.