Batter: Melt the butter in a small saucepan or place it in a stirring bowl and melt it in the microwave. Add brown sugar and vanilla extract. Whisk in the egg. Add flour and salt, combine. Stir in the chocolate chips and chopped walnuts with a spoon.
Bake: Transfer the batter to the prepared tin. Bake until lightly brown around the edges but still slightly under cooked in the middle. Let cool in the pan.