• Preheat the oven to 325°F. Line a jelly roll pan or a 15x10-inch baking sheet with parchment paper and spray with vegetable spray - spray underneath it too so it doesn't move.
• In a small bowl whisk the flour, salt, and baking soda together.
• In a separate medium-sized bowl beat the egg whites until frothy.
• Add the sugar, cocoa powder, oil, and vanilla to the egg whites and beat on medium speed until smooth and combined.
• Add the flour mixture and beat on low until combined. Stir in the chocolate chips.
• Scrape the batter onto the pan and spread from edge to edge with a rubber spatula or frosting spatula - the batter will be thin.
• Bake 25 minutes. Cool in pan on wire rack.

Make the topping:

• Place the chocolate chips in a small heat-proof bowl.
• Heat the heavy cream until it starts to bubble. Pour it over the chocolate and cover the bowl tightly with aluminum foil. Let it sit five minutes.
• Whisk the chocolate and heavy cream together until smooth and creamy. Pour onto the brownie and spread from edge to edge.
• Top with the peanut butter cups. Press them down into the chocolate gently. Sprinkle with a little coarse salt if desired. Refrigerate 20 minutes or so to help the chocolate set up.
• Remove to a cutting board using the parchment and cut into squares with a large chef's knife. Wipe it off between slices for clean cuts.
• Store in a covered container in a cool, dry place. Best eaten within 36 hours - after that the brownie starts to soften and is still delicious just not as chewy.