Preheat the oven to 160°C/fan 140°C/gas mark 3. Place the golden caster sugar in a saucepan with 140ml water over a medium heat and simmer gently until the sugar has dissolved. Add the clementine slices and the rosemary and bring to the boil, then cook rapidly for 5 minutes. Turn the heat off and leave the clementine and rosemary to sit in the syrup.
Line and grease 2 x 18cm loose-bottomed sandwich tins. Mix the soy milk and lemon juice in a jug and set aside to curdle. Sieve the flour, cacoa and bicarb into a large mixing bowl. Stir through the sugar, then make a well in the centre and pour in the soya milk mixture, oil and clementine zest. Whisk everything together until just combined, trying not to overwhisk. Split the mixture between the two tins and bake for 25-30 minutes or until firm to the touch. Once cooked, remove from the tins and leave on a wire rack until completely cool.
Place the sugar and aquafaba into the bowl of a freestanding mixer fitted with the whisk attachment. Whisk together for 5-10 minutes or until stiff peaks form – be patient. Add the vegetable fat and margarine chunk by chunk and mix for a further 10 minutes, until glossy and combined. Fold through the vanilla extract.
Remove the sprigs of rosemary from the clementine syrup and shake off any excess. Dip the sprigs in to the caster sugar to give them a crystalized coating. Drain the clementines and set aside on a wire rack.
Carefully slice each sponge in half horizontally. Place one half on a large plate or cake board. Drizzle over 2 tbsp clementine syrup, then spread over a ¼ of the frosting. Repeat with the remaining layers. Spread the last ¼ of frosting over the top and sides of the cake in a thin layer to create the ‘naked’ effect.
Decorate the top of the cake by placing the clementine slices around the edge in a wreath effect along with the rosemary.