To roast garlic, choose large heads. Cut the top off to expose the cloves (a serrated knife is best to do this). Place the head in a square of aluminum foil with the head facing up. Drizzle with 1 to 2 tablespoons olive oil. Seal aluminum foil and roast for 1 hour in a 350 degree oven. Let cool, separate heads into cloves and pinch each clove to squeeze out the garlic.
Place roasted garlic into a blender or food processor. Add the olive oil and 1/4 cup cream and whirl until garlic is pulverized. Add rest of the cream, sour cream, parmesan cheese, salt and pepper. Whirl to combine. Sauce will be thick at this point. Set aside. Cook pasta al dente; drain. Melt butter in a skillet. Add pasta and toss with the butter until coated. Add the sauce, cook and stir until sauce becomes thin. Simmer for 10 minutes, stirring occasionally. Serve immediately with extra parmesan cheese sprinkled on top.