1 1/2 cups all-purpose flour
1 cup superfine sugar
1 teaspoon baking powder
1/2 teaspoon baking soda
1/3 cup best-quality cocoa
1 1/2 sticks soft unsalted butter
2 large eggs
2 teaspoons good-quality vanilla extract
2/3 cup sour cream
Special equipment: 2 (each 8-inch diameter) layer tins with removable bases, buttered
6 ounces good-quality semisweet chocolate, broken into small pieces
3/4 stick unsalted butter
2 1/2 cups confectioners' sugar
1 tablespoon light corn syrup
1/2 cup sour cream
1 teaspoon good-quality vanilla extract
Sugar flowers, to decorate, optional