Cut 3 thick slices of brioche. Make a wide, deep slit in each slice to create a pocket for the Nutella, then fill each pocket generously with Nutella using a large spoon.

In a shallow dish, whisk together the egg, milk, and vanilla sugar.

Dip the stuffed brioche slices into the egg mixture. Melt the butter in a skillet, then cook the brioche slices for 1 minute on each side until golden. Your Nutella-stuffed French toast is ready!