Preheat oven to 400°F.
Combine bread and milk in small bowl and let sit for about 10 minutes, then work into a paste with your fingers.
In medium-sized bowl, combine ground beef, ground pork, parmesan, egg yolk, garlic cloves, parsley and kosher salt and pepper. Add bread and milk mixture and stir just until mixed.
Line baking sheet with parchment paper or aluminum foil. Use melon baller or rounded teaspoon to scoop balls of meat mixture and place on baking sheet. Lightly roll between palms of your hands into small balls and return to baking sheet.
Bake 12-15 minutes or until lightly browned and just cooked through. Set aside.
For mac and cheese:
Cook pasta to al denté according to package directions. Drain and set aside.
While pasta is cooking, prepare cheese sauce. Melt 6 tablespoon butter in large saucepan or Dutch oven over medium heat. Add flour and whisk to combine. Cook over medium-low heat for about 3-4 minutes, whisking, until flour mixture becomes light brown in color. Add 3 1/2 cups milk while whisking and cook over medium heat for about 5 minutes or until sauce thickens, whisking often. Lower heat to and cook for about 10 minutes, whisking occasionally.
Once sauce has thickened, remove from heat and stir in cheeses until melted. Add up to 1/2 cup more milk if sauce feels too thick.
Add drained pasta to sauce and stir to combine. Gently fold in cooked meatballs and serve immediately. Garnish with parsley if desired.