Preheat the oven to 350°F (175°C)
Grease and line an 8×8-inch baking pan with parchment paper
Melt 1/2 cup unsalted butter
Whisk in 1 cup brown sugar until smooth
Add 1 large egg and 1 teaspoon vanilla extract
Stir in 1 cup all-purpose flour
Add 1/4 teaspoon salt
Mix in 1/2 cup chocolate chips or nuts if desired
Spread the batter evenly in the prepared pan
Bake for 20 to 25 minutes
Cool completely before cutting into squares