Make the custard by whisking together milk, heavy cream, egg, and vanilla extract in a medium bowl. Set aside.

Trim crusts from bread slices and flatten with a rolling pin as thin as possible.

Spread Nutella evenly over each bread slice and roll them up, pinching the seam to secure.

Heat 2 tablespoons of butter in a frying pan over medium heat. Dip rollups in the custard, then cook until golden brown, about two minutes per side. Do this in batches, adding remaining butter as needed.

After cooking, roll the rollups in vanilla sugar or dust with powdered sugar if preferred. Serve immediately.