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1
Place a rack in center of oven and preheat to 350u00b0F.
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2
In a large mixing bowl, combine flour with potato starch, vanilla powder, salt, and baking soda and whisk gently to blend.
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3
In another large mixing bowl, combine sour cream, heavy cream, vanilla extract, and liquors and whisk gently to combine.
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4
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.
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5
After 2-3 minutes, add eggs and extra yolk, one at a time, waiting until each is fully incorporated before adding next egg.
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6
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).
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7
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.
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8
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).
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9
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.
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10
Remove cake from pan and allow to cool completely.
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11
Sprinkle with powdered sugar, serve with whipped cream and berries, if desired.
