Preheat oven to 350 F ( 175 degrees C). Grease a 8 inch square pan with butter or cooking spray and dust the pan with flour or coco powder. You can also line the pan with parchment paper for easy removal of the brownies.
In a large sauce pan or in a microwave melt the butter. Do not boil it. Remove from heat. let cool a bit and add the sugar. Mix well.
Add in the eggs and vanilla extract and mix until well blended.
Add flour, coco powder, salt, baking powder and walnuts (if using). Using a whisk or a spatula mix until well combined.
Pour the batter in the prepared cake pan. Bake for 25 minutes until a tooth pick inserted in the center comes out clean.
Remove from the oven and cool for at least 15 minutes before cutting.
Store the brownies in a air tight container for up to a week.