1) Place the bananas in a bowl and mash until smooth.
2) Place the slices of bread on a flat surface and spread each slice with some of the hazelnut spread. Spread the mashed banana over 6 of the slices and combine the slices to make 6 sandwiches.
3) Spread one side of each sandwich with some of the butter and place on the griddle, buttered-side down. Cook until golden brown. Spread the remaining butter on the bread facing up, flip over and continue until golden brown. Remove from the griddle and sprinkle with the icing sugar.