For filling, in large bowl, beat sugar, cornstarch and salt with electric mixer until free of lumps and well combined, about 1 minute. Add butter and beat until light and fluffy, about 2 minutes. Add egg yolks, one at a time, beating well after each addition. Add lemon juice and beat; mixture will look curdled. Transfer mixture to heavy-bottomed saucepan. Cook over medium-low heat, stirring constantly until thickened and boiling. Strain filling through a fine-mesh strainer; stir in lemon zest.