Preheat oven to 350° F. Lightly butter a 9-inch deep-dish pie plate or tart dish; set aside.
In a medium bowl, combine milk, flour, sugar, and egg yolks. Whisk to combine. Add butter, and whisk to combine.
In a separate bowl, whisk egg whites until soft peaks form. Gently, but thoroughly, fold egg whites into milk mixture. Pour into prepared dish, and bake until the custard is golden brown and a toothpick inserted near the center comes out clean, about 45 to 60 minutes.
Remove from oven, and cool on a wire rack about 5-10 minutes. Cut into wedges, and serve garnished a sprinkle of powdered sugar. Or, sprinkle with granulated sugar and brulee.