Line a half sheet pan with a nonstick rubber mat. Place a 25cm / 10in by 38cm / 15in by .5-cm / .25-in Plexiglas frame on top of the mat.
Preheat a convection oven to 160°C / 325°F.
Combine the nut pastes and confectioners’ sugar in the bowl of an electric mixer using a paddle attachment. Gradually add the egg yolks and eggs in 4 additions, scraping the bowl between additions.
Make a French meringue with the egg whites and granulated sugar by adding about 10 percent of the sugar at the start of the mixing process and whipping the egg whites at high speed. Once the egg whites have quadrupled in volume, pour the remaining sugar in slowly down the side of the mixing bowl. Whip the meringue to medium-stiff peaks. Fold into the nut paste mixture in 2 additions. Fold in the flour.
Pour in the clarified butter in a stream as the mixer runs on low speed.
Spread into the prepared sheet pan and bake until the cake springs back to the touch, about 7 minutes. Cool to room temperature.
Place the previously used frame on the cooled sponge. Trim its borders so the sponge is inside the frame, not underneath it.
Freeze the sponge to harden.
Once it has hardened, flip the cake over and peel off the nonstick rubber mat.
Spread an even, thin layer of melted chocolate on the sponge with an offset spatula.
Flip the cake back over. Cut out 9-cm / 3.5-in squares. Reserve frozen and covered in an airtight container.