Spread 1 tablespoon chocolate-hazelnut spread on 4 bread slices. Layer with bananas and top with remaining bread.
In a grill pan or large skillet, melt 2 tablespoons butter over medium. Cook 2 sandwiches, pressing with a heavy skillet, until bread is golden and toasted and spread is melted, 3 to 5 minutes, flipping once. Repeat with remaining butter and sandwiches. Serve warm.