Preheat oven to 300 degrees. Generously grease and flour a 10-15 cup bundt pan.

In a large bowl, beat the butter and brown sugar until fluffy. Add eggs, one at a time, beating well after each addition.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Gradually add flour mixture to butter mixture.

Beat in the sour cream and vanilla. Spoon 1 cup of the batter into a medium bowl and set aside.

Add pumpkin and pumpkin pie spice to the remaining batter and beat until combined.

Add buttermilk and cocoa powder to the reserved cup of batter and whisk until combined.

Spoon 1/3 of the pumpkin batter into the prepared pan. Spoon half of the chocolate batter over the pumpkin batter. Repeat once, then spread the remaining pumpkin batter on top. Gently swirl the batters together with a knife.

Tap the pan on the counter a few times to release any air bubbles. Bake for 55-60 minutes, or until a wooden pick comes out clean.

Let cool in pan for 10 minutes before turning onto a wire rack to cool completely.

While cake cools, make the ganache: heat the cream in a small saucepan over medium heat until it just starts to foam around the edges. Do not let it boil.

Remove from heat and add the chopped chocolate. Let stand 2-3 minutes, then whisk until the chocolate is smooth and shiny. Let stand until ganache is cooled, but still pourable.

Place the cake on a wire rack, then place the rack over waxed paper. Spoon the ganache over the cake. Allow the ganache to set before slicing and serving.