Place a rack in center of oven and preheat to 350°F.
In a large mixing bowl, combine flour with potato starch, vanilla powder, salt, and baking soda and whisk gently to blend.
In another large mixing bowl, combine sour cream, heavy cream, vanilla extract, and liquors and whisk gently to combine.
Add butter and sugar to the bowl of a standing mixer fitted with paddle attachment; slice open vanilla pod and scrape in black seeds and begin blending on low speed.
After 2-3 minutes, add eggs and extra yolk, one at a time, waiting until each is fully incorporated before adding next egg.
Continuing on low speed, alternately add dry and wet mixtures, little by little (move swiftly through this step to avoid overworking the batter, it should take no more than 1 minute).
Stop mixer, scrape down sides of bowl and along bottom and mix on medium speed for 15 to 20 seconds, until batter is consistenly smooth.
Spray a large (11 1/2 cup) Bundt pan with an oil/starch spray; fill pan three-quarters full and level it with a rubber spatula (use any leftover batter for cupcakes).
Bake 45 minutes; test for doneness by inserting a bamboo skewer into center and removing it: when skewer shows just a touch of crumbs, remove cake from oven and cool for 5 to 10 minutes on a flat surface.
Remove cake from pan and allow to cool completely.
Sprinkle with powdered sugar, serve with whipped cream and berries, if desired.