Cook the pasta according to the package directions, drain, and keep warm in a large bowl.
In a 2-quart saucepan, over medium heat, melt the butter. Add the garlic to the pan. Cook and stir until garlic is very lightly browned (1 to 2 minutes). Add the flour; continue stirring, until flour is bubbly (about 1 minute). Add the half and half. Continue stirring until the sauce thickens slightly (3 to 4 minutes). Add the Parmesan cheese. Stir until the cheese melts (30 seconds). Remove the sauce from the heat.
Stir in the sour cream until the sauce is smooth (1 minute). Season the sauce with salt and pepper.
Pour the sauce over pasta and toss to coat. Serve with additional Parmesan cheese, if desired.