Grease and line a deep 20cm cake tin with baking parchment.
Mix together the filling ingredients and set aside.
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.
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.