339 g (1 ½ cups) unsalted butter, softened
452 g (2 ¼ cups) superfine or bakers sugar (I use organic cane sugar ground fine in a food processor)
5 large eggs room temp
1 tablespoon pure vanilla extract
2 teaspoons sweet sherry cooking wine or madeira
405 g (3 ¼ cups) unbleached all-purpose flour – (I use organic unbleached flour)
25 g (¼ cup) potato starch (if using regular a/p flour – omit this and add 32 g more regular flour)
1 tablespoon baking powder
½ teaspoon baking soda
3/4 teaspoon kosher salt
296 ml (1 1/4 cups) buttermilk
8 ounces (1 cup) sour cream