Use a pastry pie crust for a flaky crust. Preheat the oven to 425 degrees F (220 degrees C).
In a large bowl, mix together the pumpkin, sweetened condensed milk, and egg yolks. Reserve whites in separate large glass or metal bowl. Back in the pumpkin bowl, stir in 1 teaspoon cinnamon, ginger, nutmeg, cardamom, and salt. In the egg white bowl, whip egg whites until soft peaks form. Gently fold whites into pumpkin mixture. Pour filling into pie shell.
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.
Reduce the heat to 350 degrees F (175 degrees C). Bake an additional 30 minutes, or until set.