Mix the sugar and cinnamon together in a small bowl; set aside.
Make the cake
Heat the oven to 350℉. Grease a bundt pan (or regular springform pan) with butter and then flour, tapping out the excess.
Whisk together the flour, spices, baking soda, and salt in a medium bowl.
In a large bowl, cream the butter, oil, brown sugar, and granulated sugar. Add the eggs one at a time, beating until each is incorporated. Beat in the pumpkin until very smooth and well combined.
Mix in the flour mixture and mix just until flour is no longer visible.
Transfer half of the batter to the prepared pan, spreading into an even layer. Sprinkle the streusel over the batter evenly. Top with the remaining batter.
Bake until a toothpick inserted into the center of the cake comes out clean, 55 to 60 minutes. Remove from oven and cool for 10 minutes in the pan on a wire rack. Flip the cake out onto the rack and cool completely.
Notes
Make it refined sugar-free. To make this cake refined sugar-free, substitute coconut sugar for the brown sugar and maple syrup for the granulated sugar.
Substitute all-purpose flour. If you wish, swap the whole-grain pastry flour with all-purpose flour.
Try pureed sweet potato. For a lovely sweet potato bundt cake, substitute 2 cups of unsweetened sweet potato puree for the pumpkin.
Other spices. You can swap the cinnamon in the streusel for another ground spice, such as ginger, cardamom, or a combination.
Add chocolate chips. Raise the chocolate ante by throwing in 1 cup of semisweet chocolate chips to the cake batter!