Heat the oven to 160C fan/gas mark 4.
Cut
 the butter roughly into pieces and cream in a food mixer until pale. 
Add the sugar and vanilla seeds and continue to mix until pale again. 
Mix in the egg yolk and the maple syrup. 
Roast the walnuts for 5-10 
minutes in the hot oven, leave to cool and coarsely chop. Turn off the 
oven.
Add the flour and chopped walnuts to the
 butter mix and combine until a dough forms. Knead this deftly and 
swiftly on a lightly floured surface.
Once the
 dough is formed, shape into a cylinder roughly 30cm in length and 5-6cm
 in width. Cut in half and roll each half in a sheet of greaseproof 
paper, twisting the ends as if for a large bonbon.
Refrigerate for at least 30 minutes (if desired, one of the halves can be frozen for another time).
Heat
 the oven again to 160C fan/gas mark 4. Line a baking sheet with baking 
parchment. As these are best eaten fresh, slice and bake only what is 
needed, then rewrap, store and return to the fridge. Cut 3mm-thick 
slices and lay in serried ranks on the lined baking sheet.
Place in the oven and bake for 12-15 minutes. Cool on a rack.
From Cooking: Simply and Well, for One or Many by Jeremy Lee (HarperCollins, £30)