Add 200 g of sugar and 200 g butter into the heat-proof bowl and place it on the pan with simmering water to melt. Then, add 200 g of good-quality black chocolate. Let all these melt for 5 or so minutes.
Take a mold and grease it with butter, then, sprinkle some flour. Remove the extra flour. It will help the sponge not to stick to the tin.
When melted, place your chocolate into a bigger bowl and let it cool down for 2 minutes. Then, crack 6 eggs and give that a really good mix. Then, add 1 teaspoon of vanilla paste. Next, add 1 tablespoon of orange zest. Add 200 g of self-raising gluten-free flour. Fold this flour into the mix. Pour this mixture into the mold. Bake the sponge for 50 minutes at 160 degrees C.
For the covering, melt 200 g of chocolate. Add 200 g of icing sugar and 200 g of soft butter into the melted chocolate. Mix all these together. Add a pinch of salt.
When the sponge is ready, cool it down to room temperature. Cover the sponge with buttercream.