In a large bowl combine the half and half and corn starch; set aside. Add the butter to a saucepan over medium heat. Add the garlic and cook for 1 minute. Pour in the half and half and cornstarch mixture and heat, stirring occasionally until thickened. Remove from the heat and stir in the parmesan cheese until melted and smooth. Stir in the pesto and add salt and pepper to taste. Serve warm. Add extra half and half or warm pasta water to thin the sauce, if desired. Makes 4 cups.