Preheat oven to 180°C, grease a 22cm round springform cake tin and line base with baking paper. In a bowl, sift the flour, baking powder and bicarbonate of soda. In a mixer, beat the butter and 250g of sugar until light and fluffy, then gradually add egg yolks. Reduce the speed and mix in the yoghurt and vanilla essence. Gently fold in the flour with a spoon. Whisk the egg whites until stiff peaks form and then fold into the mixture, pour into the cake tin and bake for 50 mins or until skewer comes out clean when inserted into the middle. Allow to cool in the tin for 10 mins before turning out on to a rack.
To make the syrup and candied lemons, dissolve remaining caster sugar in a pan with the 200ml of water and cinnamon stick. Thinly slice lemons, place in bubbling syrup and simmer on each side for 3 mins. Remove from the syrup, cool on rack. Pour the remaining syrup slowly over the cake and allow it to be absorbed before topping with the candied lemon slices.