Step 1

Preheat oven to 325 degrees F (165 degrees C). Grease an 8-inch baking dish.

Step 2

Beat egg whites in a large bowl with an electric mixer until stiff.

Step 3

Combine egg yolks and white sugar in another large bowl; beat with an electric mixer until light and fluffy, about 2 minutes. Add butter and vanilla extract; beat until smooth, about 2 minutes. Fold in flour. Beat in milk slowly. Fold egg whites gently into the batter.

Step 4

Pour batter into the prepared baking dish.

Step 5

Bake in the preheated oven until top is golden, 45 to 70 minutes. Let cool, about 30 minutes. Dust top with confectioners' sugar.