Preheat oven to 325℉. In a microwave-safe bowl, melt the butter completely, then whisk in the powdered sugar to help dissolve. Add the egg whites and vanilla extract and whisk until smooth.
In a separate microwave-safe bowl, completely melt the chocolate in the microwave in 30-second intervals, stirring with a rubber spatula in between to ensure even heating. Scrape the melted chocolate into the bowl of wet ingredients and fold in with the rubber spatula until incorporated.
Sift in the flour, cocoa powder, baking powder, espresso powder, and salt and fold until combined. Do NOT overmix!
Lightly grease a parchment-lined baking tray with cooking spray and scrape the batter into the center of the tray. Use an offset spatula to spread the batter into a single layer,¼-1/2" thick. (Note: spread to ¼" for crispy, and ½" for chewy). Bake for 25-30 minutes.
Remove from the oven and generously cover with chocolate chips. Press the chips into the brownies with the palms of your hands, then transfer the tray to a wire rack to cool completely. Once completely cooled, break the brittle into pieces as desired.
