Pour milk into a large saucepan, and bring to a boil over medium heat.
In a medium bowl, whisk together the semolina, cornstarch, 1 cup sugar and salt so there are no cornstarch clumps.
When milk comes to a boil, gradually add the semolina mixture, stirring constantly with a wooden spoon.
Cook, stirring constantly until the mixture thickens and comes to a full boil.
Remove from heat, and set aside.
Keep warm.
Step 2
In a large bowl, beat eggs with an electric mixer at high speed.
Add 1/2 cup of sugar, and whip until thick and pale, about 10 minutes.
Stir in vanilla.
Step 3
Fold the whipped eggs into the hot semolina mixture.
Partially cover the pan, and set aside to cool.
Step 4
Preheat the oven to 350 degrees F (175 degrees C).
Step 5
Butter a 9×13 inch baking dish, and layer 7 sheets of phyllo into the pan, brushing each one with butter as you lay it in.
Pour the custard into the pan over the phyllo, and cover with the remaining 5 sheets of phyllo, brushing each sheet with butter as you lay it down.
Step 6
Bake for 40 to 45 minutes in the preheated oven, until the top crust is crisp and the custard filling has set.
In a small saucepan, stir together the remaining cup of sugar and water.
Bring to a boil.
When the Galaktoboureko comes out of the oven, spoon the hot sugar syrup over the top, particularly the edges.
Cool completely before cutting and serving.
Store in the refrigerator.
