Grease six 6-ounce custard cups; set aside. Unfold phyllo dough. Place one sheet of phyllo dough on a cutting board. Spray with nonstick spray. Repeat layering and spraying remaining phyllo, keeping phyllo covered with a damp cloth and removing only one sheet at a time.
Using a sharp knife, cut the phyllo stack lengthwise into 6 strips. Then cut the strips crosswise into thirds forming 18 rectangles. Carefully press 3 rectangles into each prepared custard cup so that the entire cup is covered with phyllo. Set the phyllo-lined custard cups on a baking sheet.
Meanwhile, in a large saucepan combine brown sugar and cornstarch; add raspberries and cranberries. Cook and stir over medium heat until thickened and bubbly. Pour the cranberry mixture into phyllo-lined custard cups.
Bake in a 350 degree F oven for 25 to 30 minutes or until phyllo is golden. Cool completely on a wire rack. Sift powdered sugar lightly over phyllo cups or serve with dollops of whipped topping, if desired. Makes 8 servings.