Preheat your oven to 350 degrees. Cream together the butter and brown sugar. Beat the egg with a fork or whisk and add it and the vanilla to the butter and sugar; stir well.
Add the three dry ingredients plus the chopped black walnuts and stir well again. Grease a baking dish with 2-inch-high sides, then pour in the batter and spread out smooth with a spatula.
Bake for 45 minutes, testing for doneness by pressing your finger on the top; when the finger-dent has some spring in it, remove the pan from the oven and let cool before serving. The brownie will “fall” from oven height and become very chewy during cooling.
Cut into squares and sprinkle on a little of the aforementioned sugar, if you like. You’re going to love these, and will probably want to double the recipe in the future. (You can experiment with heat settings and length of baking until you get it just right to suit you, if you don’t like this consistency.)