Step 1

Heat oven to 160C/140C fan/gas Grease and line a deep 20cm cake tin with baking parchment. Mix together the filling ingredients and set aside.

Step 2

Put butter, sugar and vanilla in a bowl and whisk until pale. Beat in the eggs, then fold in the flours, buttermilk and apples. Spoon half the mixture into the tin. Scatter over filling, leaving a 2.5cm clear border all around the edge. Spoon remaining mixture on top, starting by dolloping round the edges, then moving into the middle. Bake for 1 hr 25 mins or until a skewer poked in comes out clean. Cool in the tin.

Step 3

Mix the icing sugar with enough lemon juice to make a runny-ish icing. Remove cake from the tin, drizzle over the icing, then scatter with crushed sugar cubes and lemon zest.