Make the mac and cheese: Bring a large pot of salted water to a boil. Add the pasta and cook as the label directs until al dente. Drain the pasta well and let cool.
In a medium saucepan, heat the milk and 2 cups heavy cream over medium heat until warm (do not boil). In a separate medium saucepan, melt the butter over medium heat. Add the flour to the butter and stir until incorporated, about 5 minutes, making sure it doesn't brown. Add the warm milk and heavy cream, in batches, and whisk until incorporated. Bring to a simmer and cook, whisking, about 15 minutes.
Add the American, gruyere and parmesan cheeses to the milk mixture and whisk until melted, 2 to 3 minutes (do not boil). Remove from the heat. Puree with an immersion blender until smooth (or transfer to a regular blender in batches and puree). Let cool to room temperature and mix with the pasta.
Make the hot dogs: Lightly oil a grill pan and preheat over medium-high heat. Grill the hot dogs until slightly charred, 5 to 7 minutes. Reheat the mac and cheese in a saucepan with the remaining 1/4 cup heavy cream; season with salt and pepper. Place the hot dogs on the buns and top with the mac and cheese. Season with pepper.
Recipe courtesy of Marc Murphy for Food Network Magazine