Place the walnuts in the bowl. Turbo pulse once. Remove the walnuts and set aside.
Place the butter, sugar, milk, maple syrup and vanilla in the bowl. Cook on Speed 1 for 3 minutes at 60C.
Add the flours, egg, baking powder, bicarbonate soda and salt.
Blitz for 20 seconds on Speed 5.
Add the roughly chopped walnuts.
Blitz for 5 seconds on Speed 5.
Scrape the batter into the prepared cake tin. It will seem quite wet, which is fine.
Scatter over the extra walnuts.
Bake for about 40 minutes, or until the cake is well risen and a skewer inserted in the centre of the cake comes out clean.
Allow to rest in the tin for 5 minutes, then remove from the tin and allow to cool on a rack.