Preheat oven to 350ºF (180°C).
Grease a 10 or 12-inch bundt cake pan (26cm) with 2 tablespoons shortening, making sure every nook, cranny and angle is covered. Dust with flour, shaking off excess. Reserve.
Sift together 2 ¼ cups all-purpose flour, 2 teaspoons baking powder, 2 teaspoons baking soda, 3 teaspoons pumpkin pie spice and ½ teaspoon salt in a bowl.
Beat 3 eggs for a minute with an electric mixer in a large bowl (or the bowl of a stand mixer with the paddle attachment), until frothy.
Add ¾ cup sugar and ¾ cup light brown sugar and beat for 1 or 2 minutes, until thick and pale-colored.
Gradually add ¾ cup vegetable oil and beat for a minute more, until well incorporated.
Add 1 ½ cups pumpkin pureeand mix well. If using ½ teaspoon orange zest add it now.
Add flour mixture in two additions beating only until well mixed. It's important that you don't overbeat the pumpkin mixture at this point.
Pour batter into prepared pan and smooth the top.
Bake for about 50 minutes, or until a cake tester or toothpick inserted in the middle of the cake comes out clean. If the top is browning too quickly before it's done, loosely cover the cake with a large piece of aluminum foil so the top doesn't continue to brown while the inside bakes fully.
Let cool for 15-20 minutes in the pan on a wire rack. Grab the pan with kitchen towels and starts shaking it a little, until the cake moves and you make sure the bottom and sides are not stuck.
Invert onto a wire rack, and cool completely before frosting.
Add the cream cheese frosting and decorate with some orange zest and ground cinnamon.