HOW TO MAKE :

1. Combine the flour, sugar, powdered milk, and cocoa powder in a bowl. Mix well.
2. Add the egg, water, and salt. Stir until well combined.
3. Strain the batter and mix again until smooth.
4. Cover the batter and let it rest for 1 hour. Set aside.
5. Prepare the vanilla custard filling.
6. After 1 hour, stir the batter again until well combined.
7. Cook using a non-stick skillet over low heat.
8. Pour a little cooking oil into the pan and wipe it with a tissue.
9. Once the pan is hot, pour the batter according to your preference.
10. Cook until done, then remove from the pan.
11. Pour the custard into the martabak, fold it while still warm, and press the edges together.
12. Seal the edges tightly.

HOW TO MAKE VANILLA CUSTARD FILLING :

1. Combine the sugar, egg, egg yolk, salt, and milk in a bowl.
2. Mix until well combined.
3. Add the cornstarch (dissolved in a little milk first) and vanilla extract.
4. Stir until well mixed.
5. Cook over low heat until thickened.
6. Add the butter and stir until fully incorporated.
7. Transfer to a piping bag.

Yields for 15 pieces.