Preheat oven to 425 degrees. Thaw phyllo sheets; melt butter. In a bowl, combine bananas, sugar, and chocolate.
Lay 1 phyllo sheet on a clean work surface. Brush with butter; sprinkle with sugar. Top with another sheet. Repeat with remaining phyllo, brushing each sheet with melted butter as you stack them.
Form 4 triangles: Cut dough into strips with a sharp knife; place filling at one end, leaving one corner empty. Fold corner of dough over filling. Continue folding, alternating corners, making sure no filling spills out the sides. When you reach the end, trim excess dough from triangle with knife. Brush surface with butter.
Bake on a baking sheet until golden brown, 10 to 15 minutes. Cool slightly; serve.