Pour Lucky Charms cereal and whole milk in a bowl and place in the fridge for about 30 minutes. After 30 minutes, remove from fridge and pour through a fine mesh sieve sitting over a medium saucepan, pressing cereal with a wooden spoon to extract as much milk into the saucepan as possible.
Place saucepan over medium heat. Add sugar and salt and stir constantly, until the mixture just reaches the point of simmer.
While the milk is heating, lightly whisk yolks in a separate medium bowl. Once milk is at point of simmer, slowly pour about half of it into the bowl with the egg yolks, whisking all the time. Then pour the egg/milk mixture back into the saucepan with the rest of the milk.
With the heat on low, continue stirring until mixture thickens up and coats the back of your spoon (it should be about 170 degrees if you're using a thermometer). Remove from heat.
Pour heavy cream into a separate large bowl. Strain the egg/milk mixture through a fine mesh sieve into the heavy cream, continuing to whisk. Cover bowl with plastic wrap, making sure plastic wrap is touching the top of the cream mixture, and move to fridge for at least 4 hours.
When the mixture is chilled, process it in an ice cream maker according to manufacturer’s directions. When ice cream is almost done processing, add in marshmallow fluff and extra cup of Lucky Charms marshmallows.
Scoop ice cream into airtight containers and place in freezer for at least 6 hours.