Preheat the oven to 350°F. Grease and flour a Bundt pan very well (shortening works best).

In a stand mixer with the paddle, beat the butter, granulated sugar and brown sugar on medium. Add the eggs one at a time.

Beat in the pumpkin and vanilla until incorporated; the mixture will look thin and slightly grainy.

In a separate bowl, whisk the flour, baking powder, pumpkin pie spice, baking soda and salt.

With the mixer on low, add the flour mixture alternately with the buttermilk, beginning and ending with flour. Gently stir in the pecans.

Pour into the pan and bake on the top rack 50-60 minutes, until a skewer comes out clean.

When the pan is cool enough to handle, trim the top level with a serrated knife, then invert onto a wire rack and cool completely upside down (about an hour).

In a medium saucepan, bring the brown sugar, heavy cream and butter to a boil over medium heat, stirring frequently. Boil about 1 minute.

Gradually whisk in the powdered sugar and vanilla until thick but smooth; it should coat the back of a spoon and drip evenly.

Immediately pour over the cooled cake in a steady, even motion; the icing sets quickly. Ice the cake on its serving stand.