1) Preheat griddle to medium-high heat. Brush griddle with vegetable oil.
2) Cut the bananas in half, lengthwise and grill, skin side down just until warm. Remove warmed banana from the skin and cut again in half.For assembly:
1) On a piece of aluminum foil, place 1 cookie. Top evenly with banana slices, marshmallows, chocolate chips, digestive biscuit crumbs and top with another cookie. Wrap foil around the sandwich and place on the grill until warmed through and melted.