Heat oven to 160C/140C fan/gas 3.
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.