1. Using an electric mixer beat the butter and sugar together until light and fluffy; gradually beat in the eggs, with a teaspoon of flour if it begins to curdle.
2. Fold in the sieved flour and cinnamon, then fold in the yogurt and apple.
3. Divide the mixture between 2 greased and lined 17.5 (7") sandwich tins and cook in a preheated oven Gas 4, 180°C for 20-25 minutes until golden and springy to the touch. Sprinkle the extra caster sugar over one of the cakes. Cool on a rack.
4. Beat the butter for the icing until fluffy and gradually add the icing sugar until smooth. Add the yogurt and mix well, do not overbeat. Sandwich the cakes together with the icing.
Tip: Try using organic Greek yogurt with honey or natural yogurt with vanilla instead of the toffee yogurt.
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