Whisk the egg, milk, sweetener/sugar and vanilla together in a shallow bowl until combined.
In a separate bowl, combine cream cheese and the (extra) sweetener/sugar. Spread both pieces of bread with the cream cheese mixture, and add the sliced bananas to one slice of bread. Top with the second piece of bread and gently press the edges down to 'seal.'
Dip the sandwich into the egg mix, and turn to coat.
Heat a non-stick pan with cooking oil spray over low-medium heat.
Fry sandwich for about 3 - 5 minutes on both sides or until golden brown and cooked through (you can cover with a lid to speed up cooking time as long as it's on LOW heat).
Caramel Sauce:
Melt the butter in a small saucepan. When bubbling hot, add the sugar and mix well to combine. Bring to a gentle rolling simmer and slowly add the milk. Stir and allow to simmer for about 3 -4 minutes, or until the sauce begins to thicken. Add water if needed, whisk well and drizzle over the french toast.
Notes
Be sure not to soak the sandwich in the egg wash for too long or the bread will go soggy and not cook well, just enough to lightly coat the entire outer surface.