In a medium bowl stir together sugar, flour, and salt. Add eggs and milk. Beat with an electric mixer until blended. Batter will be very thin. Melt butter in the baking pan. Turn the pan to be sure all of the sides are coated then stir the remaining butter into the batter.
Pour batter into the baking pan and bake for 30 minutes. It will puff up when baking, and flatten when cool.Cut into squares and serve.