Individual Grape and Vin Santo Cakes

Name: Individual Grape and Vin Santo Cakes
Ingredients: 1 1/2 cups plus 1 tablespoon all-purpose flour, divided; 1 1/2 teaspoons baking powder; 1/2 teaspoon baking soda; 1/4 teaspoon salt; 1 stick unsalted butter, softened; 2/3 cup plus 2 tablespoons granulated sugar, divided; 2 large eggs; 1 tablespoon grated orange zest; 2/3 cup Vin Santo or other sweet wine; 1 1/4 cups seedless red grapes (7 ounces); a jumbo/Texas muffin pan with 6 large (6-to 8-oz) cups (see cooks' note, below); confectioners sugar