Preheat oven to 325 degrees Fahrenheit. Grease a 13×9 baking dish.
Separate eggs. In a stand mixer, beat the egg whites until stiff peaks form. While the egg whites are beating, make the egg yolk mixture.
In another bowl, mix egg yolks with sugar until fluffy. Add butter and vanilla extract and mix again.
Add flour and mix again. Slowly add the milk and mix again.
Slowly add the egg whites into the mixture in 3 increments. Use a whisk to gently fold in the egg whites. Do not overmix. You still want to see the egg whites on top.
Bake for 40 minute until the middle is set but still jiggly. Cool completely for about 2 hours.