How to Make It
Heat oven to 350°F. Coat a 9x13-inch baking dish with cooking spray. Line the pan with 2 criss-crossed pieces of parchment paper, leaving an overhang on all sides; coat the parchment with cooking spray.
In a medium bowl, whisk together the flour, cocoa powder, baking powder, and salt.
Using an electric mixer, beat the sugar and butter on medium-high until fluffy, 2 to 3 minutes. Beat in the chocolate and vanilla. Add the eggs, one at a time, followed by the egg white, scraping down sides of the bowl as necessary. Add the milk and beat until combined.
Reduce mixer speed to low. Gradually add the flour mixture and beat just until combined (do not overmix).
Spread the batter in the prepared dish and bake until a toothpick inserted in the center comes out clean, 30-34 minutes. Cool completely in the pan.
Holding both sides of the parchment overhang, lift the brownies out of the pan, transfer to a cutting board, and cut into 16 squares. Store the brownies in an airtight container at room temperature for up to 5 days.