Preheat oven to 425˚. Slice butter into thin pieces for faster melting and place in a 2-quart baking dish. Place baking dish in oven while it preheats, allowing butter to melt while recipe is prepared.
Place eggs, milk, sugar, and salt in a mixing bowl and beat on medium speed or mix with a whisk. Add flour until blended, then pour immediately into the preheated baking dish with the melted butter in the bottom. (Butter will come up around edges and mix somewhat with egg mixture.) Bake at 425˚ for 21-23 minutes. Remove and allow to cool 5 minutes. Cut into squares and remove from pan with a spatula. (Recipe will puff up when baked, but will fall and flatten as it cools.)