Mix the flour and xylitol in a large bowl. Add the yeast and salt (Paul Hollywood recommends adding them to separate sides of the bowl to keep the salt from 'retarding the yeast'). Add the rest of the dough ingredients, mix well, and knead by hand or in a stand mixer for 6-10 mins (if using spelt flour, make it the full 10 mins), until the dough is soft and smooth. If it starts to look sticky, add a little more flour. If it's struggling to get smooth and looks dry after a few mins kneading, add a little more milk. Form a ball and allow to rise in a lightly oiled, covered bowl for at least 1 hour or until doubled in size.

Make the filling by blitzing the walnuts, vanilla extract, xylitol and cacao powder together until the texture of ground almonds or flour. Melt the butter and milk together in a pan over low heat and pour into the food processor with the walnut powder. Add the egg yolk and pulse until it forms a slightly gritty paste.

Roll out the dough on a floured surface as thin as you can (about 2mm is great) to form a large rectangle. Brush with the tablespoon of melted butter and then carefully spread the paste over the surface of the dough (if the paste is clumping and tearing the dough, add a little milk to thin it out and make it easier to spread). It doesn't have to be perfect - just get it spread over as much of the dough as you can. If it doesn't look very long, give the dough a (very!) gentle stretch.

Starting with the wider end of the rectangle closest to you, roll the dough up like a Swiss roll. Gently lift it into a lightly greased 1kg (2lb) loaf tin and form a "U" shape in the bottom of the tin, then turn the rest of the dough to rest on top (if it is long enough, try to form another "U" on top of the dough - don't cut it, just keep snaking the dough round - but if you only have enough to stretch it over the top of the "U" below in one line, that's fine!).

Cover the loaf tin (or place inside a plastic bag) and allow to rise for another hour. For the last 20-30 mins of rising time, preheat the oven to 180C/350F/gas 4.

Brush the loaf with the egg white and bake for 45 mins - 1 hour, or until golden brown and making a hollow sound when you tap the base of the loaf. If it starts to get too dark at any point, cover the loaf with foil for the rest of the cooking time. Allow to cool before serving. Best enjoyed on the same day as baking, but will keep for another 2 days in an airtight container at room temperature.