Preheat the oven to 425°F. Line the bottom of a 12x17in baking sheet with parchment paper. Set aside.
Sift the almond flour, powdered sugar, and cake flour into a bowl. Whisk until combined. Add the whole eggs and beat on medium until well combined and fluffy. Set aside.
Add the cocoa powder to the melted butter. Stir until well combined. Set aside to cool slightly.
In a stand mixer, whip the egg whites to soft peaks. Then begin adding the sugar 1 tablespoon at a time. Continue whipping on high until you reach firm peaks. Do NOT over whip the eggs. Just short of firm peaks is fine too.
Add the chocolate mixture to the almond flour mixture and beat until combined. Gently fold in ⅓ of the whipped egg whites. Once that is mostly combined, add another third, then the final bit of egg whites. Fold gently until the mixture is no longer lumpy and is just combined.
Pour the batter into the prepared baking sheet. Use a spatula or an offset spatula to gently spread the batter into an even layer.
Bake for 8 minutes, or until the sponge springs back when touched. Cool in the baking sheet for 15 minutes. Then place a sheet of parchment paper on top of the joconde. Place one large or two smaller cooling racks upside down onto the baking sheet, then invert so that the baking sheet is upside down on the cooling racks. Remove the baking sheet and gently peel off the parchment paper and set it aside for later.
Let the joconde cool, upside down, on the cooling racks for an hour, or until completely cool. Then place the parchment paper back on the bottom of the sponge. Place the baking sheet back over the sponge and invert the cooling rack so that the sponge is back in the baking sheet.
Gently remove the parchment paper and set aside. Sprinkle the top of the joconde with an even layer of sugar, about 2- 3 tablespoons. Use a pastry brush to even out the sugar. Gently top with the parchment paper and cover with cling wrap. Chill in the fridge overnight, or at least for several hours before assembling your cake.