Heat the milk and butter together in a small pan and remove from the heat once the butter has melted, then set to one side and allow to cool until tepid.
Secure the dough hook to the mixer and place the flour, salt, sugar and yeast in the bowl, keeping the yeast apart. With the mixer running, pour the liquid into the dry ingredients and crack in the egg. Leave the machine to knead the dough for about 8 minutes, after which the dough will be soft and a little sticky. If it looks and feels overly sticky and is still clinging to the sides of the bowl, add a few dessertspoons of flour, one at a time, kneading continuously until it is less so.
Lightly oil a large bowl and place the dough into it. Cover tightly with clingfilm (plastic wrap), then leave to rise for at least 1½ hours in a warm place, or until doubled in size.
To make the filling, beat the sugar and spices into the softened butter until evenly mixed, then set aside.
When the dough has doubled, transfer to a work surface that has been dusted with flour. Roll out to a large rectangle, measuring about 45 x 35cm (18 x 14 inches). Trim the edges to neaten, then spread the spiced butter over one half of the dough, working lengthwise. Fold the other half down, covering the filling, then press down to secure. Gently re-roll the rectangle, neatening any uneven edges as you go.
Use a sharp knife or cutting wheel to cut the dough into long strips about 1cm (½ inch) wide. Twist each strip to resemble a rope, then roll up and twist to form a knot, tucking the loose end underneath. Place each bun into a muffin tray (or on to a pre-lined baking sheet) and cover loosely with clingfilm (plastic wrap). Leave to rise for about 30–60 minutes in a warm place, or until doubled in size. Preheat the oven to 200°C fan/220°C/425°F/gas 7. Make the glaze by beating together the egg yolk and milk and brush each bun evenly. Sprinkle over the sugar nibs and bake for 12 minutes, or until beautifully golden and cooked through, testing one before removing them from the oven. Leave to cool upon wire racks before eating, if you can resist!