Snap the chocolate and melt it in a heatproof bowl with the butter and a pinch of salt over a pan of gently simmering water (do not allow the bottom of the bowl to come into contact with the water) until smooth and glossy, stirring occasionally
Whisk the eggs and sugar in a stand mixer on a high speed until light, fluffy and tripled in size
Grease a 20cm/8" round cake tin all over with a little butter and line the base and sides with greaseproof paper. If you're using a spring-form tin, also wrap the outside if the tin with tin foil to prevent the water seeping into the tin
Remove the chocolate from the heat, allow to stand for a couple of minutes prior to pouring it into the egg mixture
Whisk on a medium speed to combine, then pour the batter into the lined tin
Give the tin a few gentle taps on the work-surface to bring the air bubbles to the surface, then place the tin into a shallow roasting tray
Place the tray into the oven and carefully pour enough boiling kettle water into the tray to come halfway up the side of the cake tin
Gently slide the shelf back in and bake for 35-40 minutes, or until set but with a slight wobble
Squeeze the clementine juice through a sieve into a small pan and simmer over a low heat for 20 minutes, or until syrupy enough to coat the back of a spoon
Carefully pour it into a bowl and leave to cool
Remove the torte from the oven and leave to cool in the water-filled tin for an hour
Gently loosen the edges with a butter knife, then carefully turn it out onto a serving board/plate and peel off the paper
Slice and serve with a drizzle of the clementine syrup