1. Preheat oven to 350 degrees Fahrenheit. Grease and line an eight- by eight-inch baking pan.
2. Mix melted butter with brown sugar until smooth. Beat in egg and vanilla. Add flour and salt and mix in your choice of add ins.
3. Pour into pan and bake for 20–25 mins. Don’t overbake; they’re better chewy than dry.
4. Let cool and cut into half-inch bars.