2 1/3 cups all-purpose flour
1 teaspoon baking powder
1/2 teaspoon salt
1/4 teaspoon baking soda
1 stick unsalted butter
3/4 cup granulated sugar
2 large eggs
1 1/2 teaspoons grated grapefruit zest
1 1/2 teaspoons grated lemon zest
1/2 cup chopped candied orange peel
2 tablespoons unsalted butter
1 vanilla bean, halved lengthwise and crosswise, seeds scraped out
1 cup confectioners' sugar
3 to 4 tablespoons heavy cream
Silver nonpareils, for decorating