Combine ingredients from milk to cocoa powder in a saucepan. Cook on low flame till warm and smooth, do not boil.

In a bowl, whisk the egg till foamy and keep aside.

Take the mixture off the flame, slowly pour the egg into it, while continuously whisking till fully incorporated. Make sure to whisk well so that the egg doesn't curdle.

Keep back on the flame and cook till the custard thickens and becomes glossy.

In a baking tray, smear butter or cooking oil. Line the croissants. Poke the croissants lightly with a fork.

Pour the custard all over and press the croissants in lightly. Allow it to sit for 15 minutes to soak up.

Preheat oven to 180 degrees. Sprinkle the chocolate chips at the top. Bake for 30 minutes. Allow to cool slightly.

Serve warm.