Preheat the oven to 350-degree F or 180-degree C for at least 10 minutes.
First melt the butter in microwave or on stove top. And let it cool while you prepare baking pan and other ingredients. I recommend using unsalted butter, if using salted butter then skip the salt in the recipe.
Lightly butter or spray with non-stick cooking spray 8x8 inch baking pan. Line it with parchment paper with sides up. So it will be easy to lift up after it bakes. Again butter or spray the parchment paper.
Take all purpose flour, sugar, salt and baking powder in a bowl.
Now sift the cocoa powder. And because of that there will be no lumps of cocoa powder.
Mix well till incorporated well.
Now take melted and cooled butter in another bowl. Add yogurt and vanilla extract.
Whisk well till everything gets mixed.
Add dry flour mixture. Mix well. Do not over mix.
Add chocolate chips or walnuts if using. And mix well.
Now pour the batter into prepared pan.
And bake it into pre-heated oven for 35-40 minutes. Check by inserting a toothpick in the centre, if it comes out clean then it is ready. Let brownie cools completely in the pan on cooling rack before cutting. It takes at least 30 minutes.
After it cools completely, remove it to the cooling rack with the help of the parchment paper.
And using a sharp knife cut into 16 pieces or as per your desire.
Dust them with powdered sugar (optional) and serve.