Thaw 6 phyllo sheets; melt unsalted butter. In a bowl, bananas, sugar and chocolate chunks.
Lay 1 phyllo sheet on a clean work surface. Brush with butter; sprinkle with sugar. Top with another sheet. Repeat with remaining phyllo.
Forming Triagles; Brush each sheet with melted butter as you stack them. Cut into strips with a sharp knife. Place filling at one end, leaving one corner empty (bottom left). Fold corner of dough over filling (left). Continue folding, alternating corners, making sure no filling spills out the sides. When you reach the end, (top right), trim excess dough from triangle with knife. Brush with butter.
Bake in a baking sheet until golden brown, 10-15 minutes. Cool slightly, serve.