For filling, whisk grapefruit juice and cornstarch in medium saucepan until dissolved. Add sugar, egg yolks, zest and salt; whisk until well combined. Cook, stirring constantly, over medium-low heat until mixture thickens and boils, about 7-8 minutes. Reduce heat to low and continue cooking and stirring for 30 additional seconds. Remove from heat and stir in butter until melted and completely mixed in. Spread evenly over crust. Place plastic wrap directly on filling; cool completely.