Pre-heat the oven to 355 ºF (180 ºC) and line a 10 x 15 inch (25 x 38 cm) baking tray or sheet with baking/greaseproof paper.
In a heat-proof bowl above a pot of simmering water, melt the chocolate and butter together smooth and glossy. Set aside and allow to cool slightly.
In a stand mixer with the whisk attachment, or with a hand mixer with the double beater attachments, whisk together the eggs and sugar until pale, fluffy and about tripled in volume.
Add the melted chocolate and whisk for a few seconds until fully incorporated.
Sift together the dry ingredients (gluten free flour blend, xanthan gum, cocoa powder, baking powder, baking soda and salt), and fold them gently into the fluffy chocolate mixture, until no flour clumps remain.
Transfer to the lined baking tray and bake at 355 ºF (180 ºC) until an inserted toothpick comes out clean (about 10 - 12 minutes).
Once baked, leave the sponge in the baking tray for 2 minutes, then turn it out of the baking tray onto a piece of baking paper generously dusted with powdered sugar. Peel away the top layer of baking paper, and roll up the sponge. Leave it cool completely.