1. Prepare filling: Preheat oven to 400°. Bake sweet potatoes on a baking sheet 1 hour or until tender. Reduce oven temperature to 350°. Let potatoes stand until cool to touch (about 20 minutes); peel and mash with a potato masher.
2. Beat mashed sweet potatoes, 2 Tbsp. softened butter, and next 5 ingredients at medium speed with an electric mixer until smooth. Spoon mixture into an 11- x 7-inch baking dish coated with cooking spray. Bake at 350° for 30 minutes. Remove from oven; let stand 10 minutes.
3. Beat egg whites at high speed with an electric mixer until foamy. Gradually add granulated sugar, 1 Tbsp. at a time, beating until stiff peaks form and sugar is dissolved. Spread meringue over sweet potato mixture; bake 10 more minutes or until golden.