Chewy Chocolate Brownie Bark

Preheat the oven to 325 °F. Line a baking sheet or jelly roll pan with parchment paper. In a small bowl, whisk together the flour, salt and baking soda. Set aside. In a separate medium bowl, whisk the egg whites until they're light and frothy. Add the sugar, cocoa powder, espresso powder (optional), oil and vanilla, whisking until smooth. Whisk the dry milk powder into the egg mixture, then whisk in the flour mixture, just until combined. Stir in 1/2 cup of chocolate chips. Pour the batter onto the parchment paper. Using an offset spatula, spread the batter as thin as possible, and then sprinkle the remaining chocolate chips atop the batter. Bake the bark for 20 minutes. Remove it from the oven, and using a sharp knife, cut it into squares. (Do not separate the pieces.) Return the bark to the oven and bake it for 5 more minutes. Remove the bark from the oven and allow it to cool completely on the pan before breaking apart the pieces.