Start by taking two slices of bread and spread a generous layer of Nutella on the insides of each slice. Close the slices like a sandwich, creating two Nutella-stuffed sandwiches.
In a shallow bowl, whisk together the eggs, milk, vanilla extract, and cinnamon until fully combined.
Preheat a non-stick skillet or griddle over medium heat, and add the butter, allowing it to melt and coat the surface.
Dip one stuffed sandwich into the egg mixture, ensuring both sides are well-coated but not overly soggy.
Place the sandwich on the skillet and cook for about 3-4 minutes on one side, until golden brown, then flip and cook for an additional 3-4 minutes on the other side.
Repeat with the second sandwich, adding more butter to the skillet if necessary.
Once both sandwiches are cooked, remove them from the skillet and let them cool slightly before slicing them diagonally.
Dust the stuffed French toast with powdered sugar and serve warm with fresh berries on the side, if desired.