Preheat the oven to 325°F. Line a baking sheet with lightly-greased parchment or with nonstick aluminum foil., In a small bowl, whisk together the flour, salt, and baking soda. Set the mixture aside., In a medium bowl, working with a whisk or an electric mixer, beat the egg whites until foamy. While whisking, gradually stream in the sugar; continue to whisk until the mixture is very shiny and falls back into the bowl in thick trails that stay suspended on the surface ("ribbon stage")., Add the cocoa powder, espresso powder, oil, and vanilla, beating until smooth. Whisk in the flour mixture until no dry streaks remain., Transfer the batter to the prepared pan and spread it into a rectangle about 10" x 13". Sprinkle the chocolate chips evenly atop the batter., Bake the brownie brittle for 20 minutes. Remove it from the oven, and cut it into squares using a knife, bench knife, or rolling pizza wheel; don't separate the pieces., Return the pan to the oven, and bake the brownie brittle for an additional 5 minutes. Remove it from the oven and let cool completely on the pan. Break the cooled brownie brittle into pieces; serve at room temperature for maximum crispness., Store leftover brownie brittle airtight at room temperature for up to 5 days.