Prepare a cool oven (325°F, 160°C, gas mark 3). Grease a round 8" cake tin. Line base and side with greaseproof paper; grease paper.
Place oil, yoghurt, syrup, caster sugar and eggs in a bowl; beat with a wooden spoon until well mixed. Sift flour, cocoa, bicarbonate of soda and salt into bowl; mix well.
Pour mixture into prepared tin and bake in centre of oven for 1 hour 30 minutes to 1 hour 40 minutes. Test with the fingers. If cooked, cake should spring back and have begun to shrink from the side of the tin.
Leave cake to cool in tin, then turn out and remove paper.
Cream butter. Add yoghurt, vanilla, icing sugar and cocoa.
This quantity is sufficient to ice and use as a filling