Preheat oven to 350 degrees. Spray a 12 indentation mini bar/dessert pan with baking spray.
Melt butter in a large saucepan set over medium heat. When melted, remove from heat and add chocolate chips. Stir until melted, then stir in the cocoa powder and sugar. Set aside and let cool to lukewarm.
Stir the flour, baking powder and salt together in a small bowl
When the chocolate/butter mixture has gone from hot to warm, stir in the cold eggs and vanilla.
Add the flour mixture and stir to combine, then stir in the walnuts. Make sure the batter is cool, then add chocolate chips if using. Pour batter into prepared baking pan and bake 16-18 or until brownies appear set. Let cool in the pan for about 15 minutes, then use a knife to carefully dislodge the bars from the pan.