First, preheat the oven to 350°. Crack the eggs and separate the whites from the yolks. Place the whites in the fridge for later.
In the bowl with the yolks, add the sugar and beat it with a hand mixer until it's smooth. Add flour, baking powder, vanilla, and milk. Mix with a hand mixer until combined.
Remove the whites from the refrigerator. Add salt and beat them with the hand mixer on high for 2-3 minutes until soft peaks form. With a rubber spatula, gently fold the egg whites into the cake batter.
Grease the bottom of an 8×8 or equivalent baking dish with the butter. Pour the cake batter into the dish and bake at 350 for 50 minutes, until a toothpick inserted comes out clean.
Let the cake cool for 10-20 minutes.