While bringing your pasta water to a boil, melt 4 tablespoons of butter in a large, heavy-bottomed pot until foaming subsides. Add pancetta and cook over medium-high heat until fat is rendered and pancetta is just beginning to crisp.
Add leeks, 1/2 teaspoon each kosher salt and black pepper to the pot. Cook until leeks are softened, about 5 minutes. Reduce heat and continue cooking, stirring often, until leeks are very soft and caramelized, about 15 minutes longer. Stir in flour and cook for 1 minute. Add milk and half and half, stirring until sauce is smooth and thickened, about 3 minutes. Whisk in Fontina, Gorgonzola, and 1/4 cup Parmigiano Reggiano. Season to taste with additional salt and pepper.