Preheat the oven to 350F. Line a large baking tray (about 18 by 13-inches) with parchment paper.
Sift together the flour, cocoa powder, salt, and baking soda in a medium bowl. Set aside.
Add the sugar, water, oil, and dark chocolate to a 3-quart saucepan over medium heat. Heat until the chocolate is melted, stirring frequently. Remove from the heat and cool slightly.
Once the sugar mixture is cooled slightly, add it to a large bowl. Whisk in the egg whites and vanilla. Add the dry ingredients, and stir until combined.
Spread the brownie batter out as evenly as you can get it on the prepared baking tray (try to get it within about an inch of the edges).
Sprinkle the chocolate chips and chopped walnuts evenly on top.
Bake until the brownies are fully set throughout, about 30 minutes, rotating the tray every 10 minutes. Be careful not to let the edges char.
If you want perfect squares, score the brownies while they’re hot. If you want shards, break them apart once they’re cool.
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Notes
Try not to let the edges burn when you’re cooking these. Rotating the pan helps a lot with this. Also, note that the brittle will harden as it cools.
If you want your brownie brittle served as squares, make sure to score it while it’s still hot. Otherwise, you can let it cool and then break it into pieces.
This is a fun recipe to play with the toppings! Any type of chocolate chips and chopped nuts will work, and you can also add sprinkles.
Store brownie brittle in an airtight container at room temperature for 7 to 10 days.