PREHEAT oven to 325F. Line a rimmed baking sheet with parchment that hangs over the edge by 1 in. Whisk egg whites in a large bowl until foamy. Whisk in sugar, butter and vanilla. Whisk flour, cocoa powder, baking powder and salt in a small bowl. Stir into egg white mixture. Pour onto prepared sheet and spread evenly. Sprinkle chocolate evenly over top.
BAKE for 20 min. Remove and cut into 24 squares. Return to oven and continue baking until top is no longer shiny, about 5 min. Let stand on a rack for 5 min. Slide parchment with bark onto rack and cool completely. Store bark in an airtight container up to 1 week.
Chatelaine Quickies: Caramel skillet s'mores
Nutrition (per piece)
Chocolate Toffee: Sprinkle bark with ¼ cup toffee chips before baking.
Chocolate Peanut: Sprinkle bark with ½ cup unsalted halved peanuts.
Chocolate Coconut: Sprinkle bark with ¼ cup shredded unsweetened coconut.