Sift sugar, cornstarch, and salt into a medium saucepan. Gradually blend in 1/2 cup cold water and lemon juice. Place saucepan over medium-low heat, and whisk until smooth. Add egg yolks and 2 tablespoons butter, and blend thoroughly. Stir constantly with a wooden spoon until thickened, and add 1 1/2 cups boiling water. Continue stirring until mixture reaches desired thickness, 2 to 3 minutes.
Remove from heat, and stir in lemon peel; rub surface lightly with 1 tablespoon butter to prevent a skin from forming. Can be refrigerated for up to 1 week.