Chocolate Frosting

• 1/2 cup (1 stick) unsalted butter
• 2 ounces unsweetened chocolate, chopped
• 1/2 cup unsweetened cocoa powder
• 1 teaspoon vanilla extract
• 2 cups powdered sugar
• 1/3 cup plain low-fat yogurt

Melt butter and chocolate in heavy small saucepan over low heat, stirring constantly until smooth. Transfer to large bowl. Whisk in cocoa and vanilla. Whisk in 1 cup sugar and yogurt. Whisk in remaining 1 cup sugar.