Place the butter, sugar, mixed spice, a good grating of nutmeg and a pinch of sea salt in a pan over a low heat and warm gently until melted, stirring occasionally.
Remove from the heat, pour in the milk, then crack in the egg and whisk well.
Sift the flour and cocoa powder into a large bowl and tip in the yeast. Make a well in the middle, then gradually pour in the milk mixture, stirring and bringing in the flour from the outside to form a rough dough.
Knead on a flour-dusted surface for around 5 minutes, or until smooth and springy.
Chop half the chocolate into 12 equal-sized pieces and put aside, then finely chop the remaining chocolate into tiny chips.
Roughly stretch and flatten out the dough, sprinkle over the chocolate chips and raisins, then roughly fold over and continue to knead for a few minutes, pushing any chocolate or fruit that escapes back into the dough.
Place in a flour-dusted bowl, cover with a clean damp tea towel and prove in a warm place for 1 hour 30 minutes, or until doubled in size.
When the dough is ready, transfer to a clean surface and use your fist to knock out the air, then knead for a few minutes until smooth again.
Preheat the oven to 180ºC/350ºF/gas 4, and grease and line a 25cm x 35cm baking tray.
Divide the dough into 12 equal pieces. Flatten one piece slightly, push a piece of chocolate into the centre, snapping it to fit, if needed, then fold the dough back over the chocolate to trap it inside. Shape the dough into a neat ball and place on the prepared tray. Repeat with the remaining dough, leaving a small gap between each bun.
Cover with a damp tea towel and leave to prove in a warm place for around 45 minutes, or until doubled in size.
Mix 2 tablespoons of flour with enough water to give you a pancake batter consistency, then transfer to a piping bag. Pipe thin crosses across the top of the buns.
Bake for 25 minutes, or until golden, then transfer to a cooling rack. Warm the honey a little to loosen, then gently brush over the buns to glaze.
Slice open while warm to enjoy the melting middle – delicious served with a little butter and a pinch of sea salt, if you like.