Position the oven rack to lower 1/3 of the oven and preheat the oven to 350 degrees F. Line a 8x8 inch baking pan with parchment paper making sure to leave a little overhang. Spray with cooking spray, set aside.

In a large microwave-safe bowl, melt the butter for about 90 seconds. Stir in the sugar, brown sugar, and cocoa powder. Make sure the butter isn't very hot before proceeding. If so, allow to cool for 5 minutes. Stir in the vanilla, eggs and egg yolk until well combined. Add the baking powder, flour, cornstarch, and stir until just combined. Pour the batter into the prepared pan and bake for 20-25 minutes until a toothpick comes out almost clean. Let cool completely in the pan on a wire cooling rack.

To make the frosting: Place the cream in a small microwave-safe bowl and heat for 1 minute. Add in the chocolate and let stand for 4 minutes before stirring. Stir the frosting till smooth. If chocolate doesn't melt all the way, zap in the microwave for 15-20 seconds, stir till smooth. Pour over the cooled brownies and smooth using an offset spatula. Sprinkle with the chopped walnuts. Refrigerate for 30 minutes - 1 hour or until frosting is set, before cutting. (see notes)