Preheat the oven to 325°. Completely line a large rimmed baking sheet (18x13 works well) with a piece of parchment paper. Do not skip this step! The recipe will not work without the parchment paper. The brittle will be ONTO the pan without it!
In a microwave safe bowl melt together the butter, vegetable oil, and 4 bittersweet chocolate squares, for about 30 seconds. Give it a good stir to make sure the chocolate is completely melted. Stir the cocoa powder into the melted chocolate mixture. Set aside.
In a stand mixer fitted with the whisk attachment (or handheld mixer), whisk the egg whites until they are really frothy, about 4 minutes. Add the vanilla extract and sugar to the egg whites and whisk another 15 seconds. Pour the melted chocolate mixture into the egg white mixture and mix for another 20-30 seconds.
In a separate bowl stir together the flour, baking soda and salt.
Pour the flour mixture into the batter and fold it in with a spatula or wooden spoon until it is combined.
Pour the batter onto the parchment paper. Using a spatula spread the batter across the parchment paper until it is really thin, about ¼ inch thick at most. It will be like you are painting the parchment paper! Make sure you get the batter spread thin enough so it bakes crispy!
Sprinkle the mini chocolate chips across the batter.
Bake for 15 minutes (if the edges aren't burning or browning, bake up to 18 minutes). Watch the pan towards the end of the cooking time so the edges don't burn.
As soon as the brittle comes out the oven it will still be soft, score it with a knife or pizza cutter (into shapes that you want the individual pieces to be). Then let it cool for about 15-20 minutes to dry and get crispy. Break it apart into bite sized pieces.
Enjoy your Chocolate Brownie Brittle!!
Keep your brownie brittle in an airtight container at room temp for up to a week.