Heat oven to 350 degrees .Lightly coat a 8 inch square pan with cooking spray.In medium bowl ,whisk together the flour, cocoa powder,baking powder , and salt.
Set mixture aside.
Heat butter in a small saucepan until melted, being careful not to burn it. Remove the pan from the heat/add chopped chocolate,stirring until melted.Let chocolated cool until tepid.
In a medium bowl, lightly whisk the egg and egg yolks just until combined.Whisk in the cooled chocolate mixture, sugar,oil,vanilla extract. Do not over mix.
Use a rubber spatula to scrape the batter into prepared pan and smooth top. Bake brownies on the center rack until edges begin to pull away from sides, about 25 minutes. The browniess will look under-baked, but they will set up more as they cool.