In a bowl, beat together the sugar, egg yolks and water using the whisk on a hand mixer for approx. 5 mins. until the mixture is light and fluffy. Beat the egg whites with the salt until stiff, add the sugar, continue beating until the egg whites turn glossy. Mix the flour and baking powder, add to the mixture in layers along with the egg whites, carefully fold in using a rubber spatula, transfer to the prepared tin.
Approx. 20 mins. in the centre of an oven preheated to 180°C. Remove from the oven, allow to cool slightly, remove the tin frame, tip the sponge out onto a cooling rack, remove the baking paper and leave to cool completely. Cut the sponge in half crosswise, drizzle both cut surfaces with clementine juice. Clean the tin, brush with a little oil, line with cling film and place on a cake plate.
Bring the water, clementine zest and juice to the boil along with the icing sugar, remove the pan from the heat. Stir in the gelatine, pour through a sieve into a bowl, allow to cool slightly.
Combine the yoghurt and sugar. Dissolve the gelatine in water, stir in 2 tbsp of the yoghurt mixture and immediately stir into the remaining mixture. Combine half of the yoghurt mixture with 8 tbsp of the clementine jelly.
Place one half of the sponge in the prepared tin, spoon the yoghurt mixture on top, chill for approx. 20 mins. Place the second half of the sponge on top, cover with the yoghurt and clementine mixture, chill for a further 20 mins., top with the clementine jelly, cover and chill for approx. 3 hrs.