Streusel Pumpkin Pie

1 (15 oz) can pumpkin puree
1 (14 oz) can sweetened condensed milk
2 egg yolks
1 tsp ground cinnamon
½ tsp ground ginger
½ tsp ground nutmeg
½ tsp salt
2 egg whites
1 (9 inch) unbaked pie shell
2 Tbs all-purpose flour
¼ cup packed brown sugar
1 ¼ tsp cinnamon
2 Tbs butter, chilled
1 cup chopped walnuts

1. Preheat oven to 425°F (220°C).
2. In a large bowl, mix together the pumpkin, sweetened condensed milk, and egg yolks. Stir in cinnamon, ginger, nutmeg, and salt. In a large glass or metal bowl, whip egg whites until soft peaks form. Gently fold into pumpkin mixture. Pour filling into pie shell.
3. Bake for 15 minutes in the preheated oven. While the pie is baking, prepare the streusel topping: In a small bowl, combine the flour, brown sugar, and 1 teaspoon cinnamon. Blend in the cold butter with a fork or pastry blender until the mixture is crumbly. Mix in the chopped nuts. Sprinkle the topping over the pie.
4. Reduce the heat to 350°F (175°C). Bake an additional 40 minutes, or until set.