Prepare the non-stick baking tin, by base-lining with baking paper and lightly rubbing the inside with a little butter.
Genoise Sponge
Pre-heat the oven to 180 C/350 F/Gas 4.
In a bowl or container, weigh and thoroughly mix together the flour, cocoa, salt and xanthan gum (if required). Set aside.
In a small heat-proof bowl, melt the butter (in a microwave (covered), set to medium on 30 second bursts). Set aside.
Get ready a large heat-proof bowl and a large saucepan with an inch or two of water (over which the bowl sits snugly, but doesn't touch the water).
Bring the water to a very low simmer.
Weigh the sugar and crack the eggs into the large heat-proof bowl and whisk continually using an electric hand whisk (set over the barely simmering water) for 7 to 10 minutes.
Keep whisking until the mixture is pale, has almost tripled in volume and a lasting trail (of a few seconds) can be formed on the surface when the whisk is lifted.
Remove from the heat, before sifting the flour-cocoa mix across the surface and gently drizzling the melted butter around the edge of the bowl.
Quickly and gently fold the dry ingredients and butter into the mixture using light and high sweeps with a large spoon (in a circular folding motion), retaining as much air as possible. Fold until the mixture is even, but be careful not to over-mix.
Pour the mixture into the prepared baking tin and very gently smooth the surface.
Bake for about 20 to 30 minutes until well-risen. The top should just spring back when pressed and a cake skewer inserted will come out clean. Keep a close eye and check minute by minute at the end to be sure it isn't over-baked.
Leave the sponge to cool in the tin for 5 to 10 minutes, before removing and transferring to a wire rack to cool completely. To ensure moisture is retained, cover with a clean tea towel while cooling.