PREHEAT oven to 450° F and grease baking sheet.
MIX mashed banana, 3 tablespoons dulce de leche and walnuts in a small bowl.
ROLL out pie dough on a floured surface and extend with rolling pin to approximately 12-inches in diameter. Cut out 3-inch circles with cookie cutter or rim of a small bowl.
PLACE 1½ heaping tablespoons of dulce de leche mixture. Brush dough circle edges with egg; fold in half and crimp edges with fork to seal. Place empanadas on prepared baking sheet; brush with egg and prick a few holes with fork.
BAKE for 12-15 minutes or until golden. Sprinkle with sugar and drizzle with remaining dulce de leche.