In medium saucepan, whisk to combine yolks, 3/4 cups sugar and lemon juice. Whisk in cornstarch mixture. Whisk gently over medium heat until sugar is dissolved and lemon mixture is hot, about 4 minutes.
Boil over moderate-high heat for 1 minute, whisking constantly until the mixture is thick and glossy.
Strain lemon mixture into a heat proof bowl and stir in lemon zest. Let cool and serve. May be stored in refrigerator up to 2 weeks.