Preheat oven to 400 degrees F.
Add butter to oven-proof skillet and place in oven to melt butter. If you don't have an oven-proof skillet, you could alternately add already melted butter into a 9x13" baking dish.
Next, make the batter.
Beat eggs in large bowl.
Add flour, milk, sugar, and salt and stir until well combined.
Carefully pour mixture into buttered skillet.
Bake in preheated oven for 30-40 minutes. If you use a 9x13" baking dish, cooking time will be closer to 30 minutes and if you use a skillet, cooking time will likely be closer to 40 minutes.