Heat the oven to 160C/fan 140C/gas 3. Put the butter and muscovado sugar in a food processor and whizz for a couple of minutes, then blend in the eggs. Add the flour and baking powder and whizz until combined. Stir in the apple and toffee pieces.

Pour into a buttered, base-lined 20cm square tin and bake for 1 hour, or until risen and dark golden. Remove from the tin and cool. To make the frosting beat the butter, vanilla and icing sugar with 3 tbsp of the toffee sauce until smooth. Spread over the cake then drizzle over more sauce to finish.