Whisk the eggs and sugar together until slightly pale and thickened then whisk in the whole milk, heavy cream and vanilla extract.
Place 4 Nutella balls on a brioche slice and top with a second slice. Place brioche sandwiches into the liquid mixture and leave for 30 seconds on each side to soak. Repeat with the remaining brioche slices to make 4 French toast sandwiches.
Butter a frying pan and place over medium heat. Place a sandwich in the pan and cook for 2 minutes on each side, or until golden brown.
Repeat with remaining ingredients. Serve with berries.