Spread 1 tablespoon Nutella over each bread slice. Layer banana over Nutella on 2 bread slices. Sprinkle with salt. Top with remaining bread slices.
Melt butter in a large skillet over medium heat. Add sandwiches to pan; cook 1 minute on each side or until golden brown. Remove sandwiches from pan; cut into triangles or strips. Dust evenly with powdered sugar.