Position the rack in lower third of the oven and preheat to 350°F Spray the pan with vegetable oil spray.
Stir together flour, cocoa, salt, and baking powder with a whisk.
Melt butter in medium saucepan. Off the heat, stir in the sugar until combined (texture will remain sandy). Add the egg, egg whites, vanilla, and dissolved espresso powder. Beat with a wooden spoon about 40 strokes, scraped side of pan as necessary.
Add the dry ingredients and beat for another 40 strokes, or until completely mixed. Scrape mixture into prepared pan. Spread evenly.
Bake 20 to 25 minutes or until a toothpick inserted into the center comes out a little gooey. Cool on a rack. Cut into 16 squares.