Place a nonstick rubber mat on a half sheet pan. Place a 25cm / 10in by 38cm / 15in by .5-cm / .25-in Plexiglas frame on top of the nonstick rubber mat.
Sift the confectioners’ sugar and the desiccated coconut with a drum sieve.
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 just under stiff peaks.
Fold the meringue into the dry ingredients.
Pour into the prepared frame and even out the batter using an offset spatula so that the batter is flush with the frame. Remove the frame.
Bake until golden brown and crispy on the surface, about 6 minutes.
Cool to room temperature, and then freeze.
Flip the dacquoise over and spread it with a thin, even layer of white chocolate. Let it set.
Place the frame used earlier evenly on top of the cake. Trim the borders of the cake so that the cake is inside the frame, not under it. Trim the top of the cake using a serrated knife so that it is flush with the frame.
Cut the cake into two 14.5-cm / 5.75-in rings. Reserve frozen until needed.