Preheat the oven to 350°. Line an 8×8” baking pan with foil and spray lightly with non-stick cooking spray.
In a microwave safe bowl, melt the butter in the microwave. Allow to cool for 5 minutes. In the bowl of an electric mixer, mix the brown sugar with the melted butter and beat until smooth. Beat in the egg and then the vanilla. Add the salt, flour and baking powder and mix until just combined. Stir in your choice of mix-ins with a wooden spoon. Pour the batter into the prepared pan and spread to level the top.
Bake for 22-25 minutes or until set in the middle. Allow to cool on a wire rack before cutting into squares.