Adjust oven shelf to middle position, preheat the oven to 350F and line a 10 x 14 in shallow baking tray with baking paper.
Lightly grease the baking paper with sunflower oil.
Sift together the icing sugar and almond flour.
Add the whole eggs and using a stand mixer with the whisk attachment, whisk on medium high speed for about 5 minutes or until pale and doubled in volume.
In a separate bowl, whisk the egg whites until frothy, then slowly add the caster sugar, whisking continuously.
Once you have added all the sugar, whisk for a further 2.5 minutes until the egg whites form stiff peaks.
Fold ⅓ of the egg whites into the almond mixture until fully combined.
Sift in the gluten free flour blend and xanthan gum and fold them in to combine.
Then, add the remaining egg whites and fold in gently, keeping the mixture as aerated as possible.
Pour the batter into the lined baking tray, smooth it out into an even layer and bake for 11 minutes or until golden brown on top and inserted toothpick comes out clean.
Cool the sponge in the tray, then loosen the edges with a palate knife.