Combine egg whites, vanilla extract and cinnamon in another bowl. Dip 1 slice bread in egg batter, then coat on both sides with cornflake crust; repeat with remaining bread slices.
Spread mashed banana on 1 side of 2 pieces of crusted bread; top with remaining 2 bread slices to make 2 sandwiches. Cook sandwiches on griddle until golden brown, 2 minutes per side.
Cut in half and serve with fresh berries, if desired.