Preheat oven to 350. Brush eight 8-ounce ramekins with oil; set aside. In a bowl, whisk together flours, cocoa, baking powder, baking soda, and salt.
In a large bowl, whisk together sugar, applesauce, and oil until smooth. Whisk in egg white and vanilla. Add flour mixture; mix just until moistened. Divide batter among ramekins; smooth tops. Bake on a rimmed baking sheet until a toothpick inserted in center of a brownie comes out clean, 20 to 25 minutes. Transfer sheet to a rack to cool completely. (To store, cover ramekins with plastic wrap, and keep at room temperature, up to 2 days.)
Assemble sundaes: Place chocolate in a microwave-safe bowl; microwave in 1-minute increments, stirring, until almost completely melted, 2 to 3 minutes. Top brownies with frozen yogurt and chocolate; serve with additional toppings.
If baking them in an 8-inch square pan, decrease the baking time to 15 minutes.