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Heat oven to 160C/140C fan/gas 3. Grease a 2lb loaf tin and line the base and ends with a long strip of baking paper.
Beat together the butter, sugar and vanilla until pale, then beat in the eggs, one by one. Tip in the flour, cinnamon and yogurt. Peel, core and chop apples into small chunks, then add to the bowl and mix everything together with a wooden spoon.
Scrape into the tin, smooth the top and scatter the walnuts down the middle. Bake on a middle shelf for 1 hr 20-30 mins until a skewer poked in comes out clean. Cool in the tin.
To decorate, put the toffees in a small saucepan with the double cream. Gently heat, stirring, until toffees have melted into a smooth caramel sauce. Cool for about 1 min while you gently turn out the cake. Slowly drizzle the toffee sauce over the top of the cake. Scatter immediately with the extra walnuts – they should stick where they hit toffee. Leave for 10 mins before serving. Best fresh but will keep in an airtight tin for 3-4 days.