Preheat the oven - Preheat the oven to 375°F (190°C).

Melt the butter - In a small saucepan, melt the butter over low heat. Set aside. - Be careful not to burn the butter while melting. Alternatively, you can microwave the butter in 10-second increments until melted.

Prepare pastry sheets - Take the phyllo/puff pastry sheets from the package and using a sharp knife, carefully cut them to fit the size of a 9x13 inch baking dish. The pastry should hang over the sides of the dish. - Work gently and avoid tearing or stretching the pastry sheets too much, as they may break. Cover the unused pastry sheets with a damp kitchen towel to prevent them from drying out.

Mix the filling - In a large mixing bowl, beat the eggs and crumble the feta cheese. Mix until well combined. - Consider using a whisk or a fork to crumble the cheese, as you want it to be crumbly but not too fine. If you prefer a richer filling, add 1/2 cup of Greek yogurt or sour cream to the egg and cheese mixture.

Layer the pastry and filling - Brush the bottom of the baking dish with the melted butter. Lay one sheet of pastry on the bottom and brush it with butter. Repeat with 5 more sheets, brushing each one with butter. Pour in the egg and cheese mixture, spreading it evenly. Continue layering the pastry sheets and butter until you have used up all the sheets. Brush the top pastry sheet with butter. - If the pastry sheets tear, don't worry. Just patch them up and continue layering. Make sure to use enough butter between the layers to ensure flakiness.

Bake the Banitsa - Pour the milk over the top of the pastry and bake in the preheated oven for 35-40 minutes, until golden brown. - The Banitsa is done when the top is golden and crispy, and the filling is set. The milk will ensure that the pastry is crispy and flaky, while the filling is creamy and savory.

Cool and serve - Let the Banitsa cool in the baking dish for 5-10 minutes. Using a sharp knife, cut it into squares or diamond shapes. Serve warm or at room temperature. - Banitsa can be kept in the fridge for up to 4 days. Reheat it gently in the oven or microwave before serving.