Stir cream, milk, cornstarch, and 2 T. sugar together in medium saucepan until smooth. Cook, stirring constantly, over medium heat until thickened and boiling, 3-4 minutes. Boil 1 minute, then remove from heat. Let stand 30 minutes to cool. Place piece of plastic wrap directly on surface and refrigerate at least 1 hour.
Heat oven to 350 degrees. Brush a little melted butter lightly over bottom and sides of 9-inch square baking pan.
Cut phyllo crosswise in half. Cover half the sheets with lightly dampened kitchen towel and set aside. Place 1 phyllo sheet in cake pan and fold over 1 edge to make it fit. Brush lightly with butter. Repeat layering and buttering remaining half of phyllo sheets.
Spoon filling onto phyllo and spread evenly almost to edge. Top with reserved phyllo, folding edges and brushing with butter as before. Refrigerate 15 minutes.
Using sharp knife, score top in diamond pattern, making cuts 2 inches apart and 1/4-inch deep or almost to filling. Brush top of phyllo with remaining butter.
Bake baklava until top is crisp, golden, and puffed, about 35 minutes. Transfer to wire rack to cool.
Heat 1/2 cup sugar and 1/4 cup water in small saucepan to boiling. Stir in lemon juice and simmer 5 minutes. Let cool slightly, then pour over still-warm baklava. Let cool completely. Cut into diamonds and arrange on serving platter.