Make the Custard Filling:

1. Heat the Milk – In a saucepan over medium heat, warm the milk until it’s hot but not boiling.
2. Whisk the Egg Mixture – In a bowl, whisk together the eggs, sugar, flour, cornstarch, and vanilla extract until smooth.
3. Temper the Eggs – Gradually pour the hot milk into the egg mixture, whisking constantly to avoid scrambling the eggs.
4. Cook the Custard – Return the mixture to the saucepan and cook over medium heat, whisking constantly until thickened, about 5-7 minutes. Once thickened, remove from heat and stir in the lemon zest (if using). Set aside to cool slightly.

Prepare the Phyllo Pastry:

5. Preheat the Oven – Preheat the oven to 375°F (190°C).
6. Prepare the Baking Dish – Lightly butter a 9×13-inch baking dish or similar-sized dish.
7. Layer the Phyllo Dough – Place a sheet of phyllo dough in the baking dish and brush it lightly with melted butter. Repeat this process with about 6-8 sheets of phyllo, layering each sheet and buttering it in between.
8. Add the Custard Filling – Pour the slightly cooled custard filling over the layered phyllo dough.
9. Top with More Phyllo – Continue layering the remaining phyllo sheets on top of the custard, buttering each sheet as you go. Use about 6-8 more sheets to cover the top.

Bake the Bougatsa:

10. Bake – Place the baking dish in the oven and bake for 35-40 minutes, or until the top is golden brown and crispy.
11. Cool and Serve – Let the Bougatsa cool slightly before cutting it into squares. Dust with powdered sugar and cinnamon, if desired, before serving.