Preheat your oven to 350 F degrees. Coat a 6-cup bundt pan with non-stick cooking spray.
Combine flours, pumpkin pie spice, baking powder, baking soda and salt in medium bowl. Whisk to mix well. Combine butter and brown sugar in large bowl and beat at medium speed until mixed well. Beat in pumpkin and egg. Reduce to low speed and beat in flour mixture. Beat in buttermilk and vanilla, just until moistened. Pour batter into prepared pan.
Bake 25 to 30 minutes, or until toothpick inserted into center comes out clean. Cook the cake in pan on wire rack for 10 minutes. Remove cake from pan and cool completely on wire rack.
For the ganache:
Melt the bittersweet chocolate slowly in a microwave-safe bowl. Heat on 50% power in 30-second intervals until chocolate starts to melt. Then remove and stir gently until completely melted. Add the juice, a teaspoon at a time, while mixing well with a whisk. Drizzle over the cooled cake.