In a mixing bowl, combine the egg yolks, cornstarch, and 1 cup of the heavy cream. Whisk to blend well. Set aside. Combine the remaining 2 cups cream, lemon juice and sugar in a large heavy bottomed saucepan over medium heat. Whisk to dissolve the sugar and bring to a gentle boil, about 10 minutes. Slowly add the egg yolk mixture, whisking constantly it thickens, about 5 minutes. Be forewarned: the mixture will break. Don't be alarmed. Pour it into a glass bowl. Press a piece of plastic wrap directly on the surface of the mixture to prevent a skin from forming. Let cool completely at room temperature. When cooled pour the mixture into the bowl of an electric mixer fitted with a wire whip. Beat a medium speed to combine the mixture. If it will not combine, warm another 1/2 cup heavy cream and slowly add it to the mixture. Whip until you have a thick and creamy custard.