In heavy saucepan, mix 1/2 cup sugar, the cornstarch and salt. Stir in juice and yolks with whisk. Cook over medium-high heat 10 to 12 minutes, stirring constantly, until mixture thickens and boils. Remove from heat; stir in 3 tablespoons butter and the grapefruit peel. Pour filling into baked crust. Cover surface of filling directly with plastic wrap. Refrigerate 2 hours 30 minutes.