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.