Yellow Sheet Cake with Chocolate Frosting

INGREDIENTS

• Cooking spray
• 1 tablespoon all-purpose flour
• 1/2 cup butter, melted
• 1 (8-ounce) carton fat-free sour cream
• 1 1/2 cups granulated sugar
• 2 teaspoons vanilla extract
• 1/2 cup egg substitute
• 2 cups all-purpose flour
• 1 teaspoon baking soda
• 1/2 teaspoon salt
• 3/4 cup low-fat buttermilk
• 1/2 cup (4 ounces) block-style fat-free cream cheese, softened
• 1/4 cup butter, softened
• 1 teaspoon vanilla extract
• 1/3 cup unsweetened cocoa
• 1 to 2 tablespoons evaporated fat-free milk
• 1/8 teaspoon salt
• 3 cups powdered sugar, divided