Melt the butter and set aside to cool slightly.
Using a hand mixer, whisk the egg whites and caster sugar in a large bowl until soft peaks form. Set aside.
Combine almond meal, icing sugar, and flour in a food processor. Blitz briefly to mix thoroughly.
Add all the eggs to the dry ingredients and blitz for 30 seconds, or until the mixture is smooth and fully homogenised.
Transfer the batter to a large mixing bowl.
Fold in the meringue in several additions, gently rotating the bowl to maintain aeration.
Spoon a small amount of the batter into the melted butter. Mix until combined, then incorporate back into the main mixture.
Pour the batter onto a 30 × 40 cm silicone tray and level with a cranked spatula. Alternatively, pipe the sponge using an 8 mm nozzle.
Bake at 200 °C for 8–10 minutes, or until lightly golden and springy to the touch.
Remove from the oven and immediately cover with a silicone mat. Flip the sponge to prevent evaporation and retain moisture.
Allow to cool completely before peeling off the silicone mat.
