Preheat the oven to 350°F. Spray an 8x8-inch baking dish with nonstick cooking spray.
In a large bowl, combine flour, sugar, cocoa powder, salt, and baking soda. Whisk to combine.
To the dry ingredients, add vanilla, vinegar, oil, and coffee. Using a spatula, mix until the ingredients are fully combined. The batter will be thick.
Use your spatula to spread the batter out to the edges of the prepared baking dish. Top with walnuts, if using.
Bake for 17-19* minutes, or until an inserted toothpick comes out clean with a few crumbs, but no wet batter. Let cool.
Topping (optional)
In a small bowl, mix together the confectioners' sugar with cocoa powder. Sift over the cooled brownies. Cut and serve.
Notes
*It is important to not overbake these brownies! Check at 17 and test with a toothpick. If the batter is not wet, remove them from the oven and let cool for a few minutes. Overbaking these brownies can result in a cake-like texture.