Preheat oven to 215C / 425F. Grease the base and sides of a swiss roll cake tin (18x13-inch) with melted butter, line the bottom (not the sides) with parchment paper and grease the parchment paper again with melted butter. Set aside.
Fit your mixer with the whisk attachment. Ensure that your mixing bowl is dry and oil-free then add in the egg whites. Whisk the egg whites on medium speed until foamy. Turn the speed up to medium-high and whisk until soft peaks formed. Add in the caster sugar, turn the speed up to high and whisk until stiff peaks formed. Scoop out the egg whites into a clean and dry bowl. Rinse your mixing bowl and dry it.
In the now clean and dry mixing bowl, add in the sifted ground almond, sifted icing sugar and eggs. Change the mixer attachment to a paddle attachment. Beat on medium speed until the mixture is light and pale in colour, about 3 minutes.
Turn the speed to low then add in the flour. Beat until the flour has been incorporated.
Using a spatula, fold in the beaten egg whites in 3 parts until no streaks of white remain.
Scoop out 1 cup of the batter into the melted butter. Fold until all the butter has been incorporated. Pour back the butter-batter into the main batter and fold until fully incorporated.
Pour the batter into the swiss roll tin, spreading it out evenly with the spatula (especially the sides).
Bake the joconde for 5 to 7 minutes until a skewer inserted into the joconde comes out clean and the top of the joconde bounced back when lightly touched.
Run a thin knife along the edges of the swiss roll tin. Invert the hot joconde on to a parchment paper. Transfer the joconde on a wire rack. Peel off the bottom parchment paper (now on top) carefully. Place the peeled parchment paper loosely on the joconde to prevent it from drying out and let the cake cool completely.
NOTES
- My swiss roll tin is a little bit smaller - 15x10-inch - hence my resulting joconde is thicker and takes a bit longer to bake (10 minutes). - Instead of pouring the melted butter directly into the batter (the melted butter will just sink to the bottom), by mixing the melted butter with some of the batter first will allow the butter-batter to be more easily incorporated into the batter.