205g / 7.23ozbrown butter, cooled to room temperature
Combine the dry ingredients in the bowl of an electric mixer fitted with the paddle attachment. Pour in the egg whites in several additions, scraping down the bowl between each addition. Mix until just combined. Scrape down the sides of the bowl. Slowly add the butter until just incorporated.
Reserve refrigerated until needed. Not only is it easier to work with when cold, but chilling also extends its shelf life.
Preheat a convection oven to 160°C / 325°F.
Spray a rectangular cake mold 5cm / 2in by 38cm / 15in by 7.5cm / 3in with nonstick cooking spray. Pipe the financier batter almost all the way up the mold.
Bake until firm to the touch at the center of the financier, about 10 minutes.
Remove from the oven and cool in the mold.
Once it has cooled, trim the crown off the financier so the top of the cake is even with the cake mold.
Take the financier out of the mold and freeze for 1 hour, then cut into slices 2.5cm / 1in by 7.5cm / 3 in. Cut a groove (1 cm / .4 in wide by 1.25cm / .5 in deep) at the right side of each slice.
Reserve in an airtight container at room temperature. Discard after service.