Preheat the oven to 375°F (190°C). Grease a loaf pan with baking spray and line it with parchment paper.

Add the butter and sugar to a medium, microwave-safe mixing bowl. Microwave the mixture 30 seconds at a time, stirring in-between each, until the butter is melted. Then, whisk the sugar and melted butter together until the sugar dissolves a bit and the butter is room temperature, about 3-4 minutes.

Add the egg, vanilla extract, and salt to the butter-sugar mixture. Stir until the mixture is completely smooth. Then, sift the cocoa powder and flour into the wet ingredients. Whisk until the batter is about 90% combined. Add the chocolate chips, nuts, or other add-ins (if using) and stir just until there are no more streaks of flour or cocoa powder.

Spread the batter evenly into the prepared loaf pan. Bake the brownies for 20-25 minutes, until the edges are set and the center is still a bit underdone. To test if the brownies are done, insert a toothpick into the area in-between the edges and center. If the toothpick has a few moist crumbs, the brownies are done.

Let the brownies cool completely. Then, use the parchment paper to remove the brownies from the pan. Slice and sprinkle with flaky salt if desired. Enjoy!