In a medium sized sauce pan combine butter, garlic, heavy cream, milk, cream cheese, salt and pepper over medium heat. Bring to a simmer and then remove from heat and stir in the parmesan cheese. Temper the egg yolk before adding into the sauce by adding a little sauce with the egg yolk and mixing it with the sauce. Add the egg yolk into the sauce, whisking constantly to prevent the eggs from scrambling. If the mixture is too thick, add more milk if needed.
Stir in cooked pasta and gently toss to combine; season with salt and pepper, to taste.