Preheat oven to 350F.
Spray a soufflé or casserole dish with non-stick cooking spray.
In a large bowl, using an electric mixer on the lowest setting, beat together the sweet potatoes and sugar.
Add egg whites, milk, cornstarch, cinnamon and vanilla. Beat until fluffy and lump-free.
Pour into prepared dish.
In a small microwave bowl, combine the brown sugar and butter.
Microwave on high, until melted, about 30 seconds.
Stir in the flour and pineapple.
Drizzle the topping over the sweet potato mixture.
Bake for 30 – 35 minutes until bubbly and golden. Serve immediately.