Preheat the oven to 350 degrees F. Coat a 6-cup bundt pan with cooking spray and set aside.
Combine the all-purpose flour, whole wheat flour, pumpkin pie spice, baking powder, baking soda and salt in a medium bowl and whisk to mix well.
Combine the butter-flavored spread, no-calorie sweetener, and brown sugar in a large bowl and beat at medium speed until mixed well. Beat in the pumpkin and egg. Reduce speed to low and beat in the flour mixture. Beat in the buttermilk and vanilla, just until moistened.
Spoon the batter into the prepared pan. Bake 25-30 minutes or until a wooden toothpick inserted in the center of the cake comes out clean. Cool the cake in the pan on a wire rack for 10 minutes. Remove the cake from the pan and cool completely on the wire rack.
Combine the confectioners' sugar and 2 teaspoons of orange juice in a small bowl, stirring until well mixed. Stir in additional orange juice, a few drops at a time, until the desired drizzling consistency is reached. Drizzle over the cooled cake. The cake can be covered in an airtight container and stored at room temperature up to 2 days.