Prepare 10 rectangular fleximolds that measure 7.5cm / 3in by 2.5cm / 1in by 2.5cm / 1in to bake the financier. These molds generally come in sets of 12. Place the fleximolds on a half sheet pan and spray lightly with nonstick cooking spray.
Preheat an oven to 160°C / 325°F.
Combine the dry ingredients in the bowl of an electric mixer on low speed using the paddle attachment. Increase the speed to medium and add the egg whites in several additions, scraping the bowl between additions. Mix until just combined. Scrape down the sides of the bowl. Slowly add the butter until thoroughly incorporated.
Pour the batter into a piping bag.
Pipe the batter into the prepared molds, four-fifths full (about 50g / 1.76oz batter per mold).
Bake until golden brown and the financier is baked through all the way at its center, about 9 minutes.
Let the financiers cool in the mold, then push the baked financiers out of the mold.
Trim the crowns off so they form clean rectangles.
Lay a rectangle on its side. Using a 3.75-cm / 1.5-in round cutter, cut out a half-circle from the top half of the financier. This is for the quenelle to eventually sit on.