1 tablespoon unsalted butter, plus more for the pans
1/4 cup toasted breadcrumbs (optional)
1/4 pound ditalini, which are essentially mini macaroni shapes, or other small, similarly shaped pasta
1 tablespoon all-purpose flour
1/3 cup whole milk, warmed
2 ounces sharp Cheddar, grated
2 ounces Gruyère, grated
1 large egg yolk
1 tablespoon heavy cream
1 teaspoon Dijon mustard (optional)
Salt and freshly ground black pepper to taste