Sift together the flours, salt and powdered sugar.
Whip egg whites on medium speed until they reach 4 times their original volume. Add cream of tartar or lemon juice, and begin to slowly ad d the measurement of granulated sugar. The whites should become light and shiny. Continue to whip until soft peaks form when a spoon is lifted from the surface
Add sifted flour mixture to meringue all at once and fold together gently with a stiff whisk until a creamy batter is formed.
Place batter on prepared half sheet pan, the batter should only be spread to about ¼”. Bake at 325 F for 8-12 minutes, until top is lightly browned and cake springs back when pressed. Set aside to cool.
For the Buttercream
1 ½ pounds granulated sugar
¼ C water
1 C egg yolks
2 pound unsalted butter (at room temperature)
3 t vanilla extract
1 pound melted dark chocolate
Place sugar and water in a saucepan; allow to boil until it reaches a temperature of 240 degrees F.
While sugar is boiling, place the egg yolks into the bowl of a mixer with a whisk attachment.
Whip on high speed until they are light and fluffy.
When sugar reaches desired temperature, lower the speed on the mixer to medium, and slowly pour the hot sugar along the side of the bowl, as to avoid hitting the spinning whisk, which will throw the sugar around the inside of the bowl and cause lumps in your final product.
When all of the syrup is added, return to high speed and continue to whip until cool.
When the yolk mix is cool, begin adding the room temperature butter, a small amount at a time (At least twelve additions). It is very important that your butter is not cold, or your icing will break. Allow the mix to accept each addition completely before adding the next.
When all of the butter has been added, incorporate the vanilla and previously melted chocolate.
Cut your half sheet of dacquoise lengthwise into three even pieces. Place the first layer on a cake board sprinkled evenly with powdered sugar, soak with frangelico liqueur, and spread with a 1/8 inch layer of chocolate buttercream.
Place the second layer of cake on top, and repeat the process until all layers are in place. The top layer should be buttercream. Drag a cake comb lengthwise across the top to make grooves. You may also ice the sides of the cake and dust these with hazelnut flour for a more finished appearance.
Slice the cake into desired portions, serve with whipped cream, and fresh bananas cooked lightly in brown sugar, rum, and cinnamon.