Preheat oven to 350°F.
In a saucepan, combine sugar, cornstarch, and salt. Add milk. Bring to a boil; cook and stir for 2 minutes or until thickened. Add the strawberries and butter; cook until strawberries are smooth. Remove from heat. Add 1/3 cup strawberry mixture to the egg yolks; return to the pan, stirring occasionally. Stir in vanilla extract and cool slightly.
In a mixing bowl beat egg whites until foamy. Add cream of tartar; beat until stiff peaks from. Fold into the strawberry mixture. Pour into a greased soufflé dish and bake for 45-50 minutes.
Garnish with fresh strawberries and dust with powdered sugar. Serve immediately.