Preheat the oven to 350. Grease an 8 inch square baking pan, or line with aluminum foil and grease the aluminum foil.
Melt the butter over low heat. Transfer to a bowl and use the electric mixer to beat in the egg and vanilla, stirring down the sides every now and then.
Add the salt, then gently stir in the flour. Pour into the prepared pan and then bake 20 to 25 minutes, or until just barely set in the middle. Its better to underbake brownies than to overbake them. Cool on a wire rack before cutting. Store covered at room temperature for no more than a day.