Place two slices of bread on a clean work surface. Spoon 2 - 3 tbsp of the leftover apple crumble into the centre of each slice.
Place the remaining slices of bread on top.
Using a large round cookie cutter or wide-rimmed glass, press firmly around the filling to cut and seal the bread into a filled pocket. Remove the excess crusts and set aside for breadcrumbs for another day.
Whisk the eggs, milk, cinnamon and vanilla together in a shallow bowl.
Carefully dip each stuffed bread pocket into the egg mixture, coating both sides without allowing the bread to become overly saturated.
Melt the butter in a frying pan over medium heat.
Cook the stuffed French toast for 2–3 minutes on each side, until golden brown and crisp.
Serve immediately with a dusting of icing sugar or a drizzle of maple syrup, if desired.