Pierce the potato several times with a knife or fork. Microwave on high for 5 minutes or until tender. Then let cool.
Halve potato lengthwise then scrape pulp into a medium bowl. Mash until smooth. Measure 3/4 cup; set aside any remaining for another use. In a small microwave-safe bowl, melt 2 oz of chocolate in microwave on high for 30 to 45 seconds, stirring once.
Preheat the oven to 350 F. Grease and flour or use non-stick spray to coat a 10 cup bundt pan. In a medium bowl, whisk together flour, pumpkin pie spice, baking powder, and salt. In a very large bowl, beat butter and sugar on medium high 2 to 3 minutes or until light and fluffy. Add eggs, one at a time, beating after each until combined. Gradually add flour mixture, beating until combined. Beat in sour cream, mashed sweet potato, and vanilla until combined. Remove 3 cups of batter to a medium bowl then stir in melted chocolate into separated batter.
Alternately spoon mounds of each cake batter into pan. Using a knife, swirl batters together. Bake for 50-60 minutes or until a toothpick inserted near center comes out clean. Let cool in pans on a wire rack for 10 minutes. Remove from pan and let cool completely.
For ganache, in a small saucepan bring cream just to boiling over medium high. Remove from heat. Add remaining chopped chocolate. (don't stir). Let it stand for 5 minutes then stir until smooth. Spoon ganache over the top of the cake and sprinkle with flaky sea salt.
You can also make this with two 9 inch loaf pans. Bake for 45 minutes or until toothpick inserted near center comes out clean.