Preheat oven to 350 degrees. Spray an 8-by-8 inch baking pan with nonstick cooking spray. In a large mixing bowl, combine flour, sugar, 1/4 cup cocoa, baking powder and salt. Add milk, applesauce and vanilla to dry ingredients to blend. Spoon batter into pan. In a medium-size bowl, combine brown sugar, remaining 1/4 cup cocoa, and hot water. Stir to blend. Pour over batter in prepared pan. Bake for 40 minutes. Remove cake from oven and cool in pan. Serve with a scoop of low-fat frozen yogurt and spoon the chocolate sauce that has formed on the bottom of the pan over the brownie as a topping.