Preheat oven to 350°F (175°C). Grease and flour an 8-inch square baking pan.
In a medium bowl, combine the flour, baking powder, cinnamon, and salt. Set aside.
In a large bowl, cream the butter, brown sugar, and white sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla and pumpkin puree.
Gradually stir the dry ingredients into the pumpkin mixture. Fold in the pecans, walnuts, and dates (if using). Spread the batter into the prepared pan.
In a medium bowl, combine the flour, brown sugar, white sugar, cinnamon, nutmeg, and butter. Cut in the butter with a pastry blender or two knives until the mixture resembles coarse crumbs. Stir in the pecans and walnuts (if using). Sprinkle the topping over the batter in the pan.
Bake for 40 to 45 minutes, or until a toothpick inserted into the center of the cake comes out clean. Cool in the pan for 10 minutes before cutting into squares.
Interesting Facts
Pumpkin is a highly nutritious vegetable and is a great source of vitamins and minerals.
Pumpkin puree can be used in a variety of recipes, including muffins, pies, soups, and more.
Streusel topping is a mixture of butter, sugar, and flour that is used to top cakes, muffins, and other baked goods.