Combine the flour, salt and butter in a large mixing bowl and use your hands to rub the butter into the flour Add the condensed milk (optional) and egg to the water and mix well
Add the flour-butter mixture to the water mixture and knead well, just until all the dry flour has been absorbed.
Transfer the dough to a bowl, cover with plastic wrap and leave to rest for about 15 minutes.
Now, transfer the dough to a work surface and knead with your hands for about 5 minutes. Add a bit of flour if the dough seems too sticky. The dough should feel moist but it shouldn't stick to your hands.
Roll the dough in your hands to form an oval shape and cut into 5 equal sized balls. The balls will be about the size of a tennis ball (2.5 inches or 6 cm in diameter). Place the balls back into the bowl, then brush each ball with the cooking oil.
Cover the bowl and let the dough rest for at least 2 hours. You can also leave the dough refrigerated overnight, but make sure to remove them from the fridge at least 3 hours before using.
To shape the roti, flatten each ball on a flat surface and thin it out by stretching out each edge as far as it will go, without breaking the dough. Press the edge onto the surface to help keep it in place. Repeat this process until you get a very thin sheet of dough.
Heat a 12 inch flat pan over medium heat and add some cooking oil. Lift the stretched roti from the surface and place it into the hot pan. Arrange the sliced banana in the middle of the roti. Quickly fold the edges of the roti over the banana to form a square, turn over the roti and cook until golden brown and crispy from both sides. Cooking time should be about 4-5 minutes
Remove from the pan and transfer to a cutting board to bite sized squares. Drizzle with some condensed milk. Enjoy while warm