Preheat oven to 425 degrees F. Lightly grease a 9x13-inch glass baking dish.
In a large bowl, mix together brown sugar, granulated sugar, salt, cinnamon, ginger, and nutmeg.
In a separate bowl, beat the eggs until slightly thickened and lemon coloredadd to the sugar mixture. Stir in the pumpkin puree and then the evaporated milk (stir only until the mixture is uniform). Pour the mixture into the prepared baking dish.
In another bowl, pour the spice cake mix. Using a pastry blender or two knives, cut the butter into the cake mix until it resembles small crumbs (the mixture will be powdery). Carefully and gently sprinkle all the spice cake mixture over the pumpkin mixture. It will float on top of the pumpkin mixture.
Bake for 15 minutes; then reduce the temperature to 350 degrees F. Bake approximately 45 to 50 minute more. If the top begins to brown too much, loosely tent with a piece of aluminum foil slightly smaller the the baking dish. Test for doneness by inserting a knife into the center of the cake. If the knife comes out with nothing sticking to it, the pie is done. Remove from oven. Cool thoroughly before serving.
Serve with whipped cream.
NOTE: Recipe may be reduced by half, using an 8-inch square baking dish.
Variation: Sprinkle 1/2 to 1 cup toasted coarsely chopped nuts over the top of the cake before baking.
Makes 12 servings.