For the dough, melt the butter in a pan over a low heat and add the cold milk, heating until just tepid. Stir in the yeast and sugar and leave for 10 minutes.
Mix together the flour and cardamom in the bowl of a free-standing electric mixer fitted with a dough hook.
Add the yeast mixture and knead with the dough hook until the dough begins to pull away from the bowl.
Place the dough in a large greased bowl, cover with oiled cling film and leave to rise until doubled (about 1-1½ hours). Take care not to over-prove the buns as otherwise they may collapse.
For the raspberry jam, add the raspberries and sugar to a large pan and bring to the boil until thickened.
Transfer to a large heatproof bowl and leave to cool. When cooled, set aside in the fridge until needed.
For the custard, warm the milk and cream in a pan until simmering.
Whisk together the egg yolks, sugar and vanilla extract in another bowl. Whisk in the cornflour.
Slowly pour the hot milk mixture into the egg mixture, whisking continuously, then return to the pan.
Cook over a very low heat, stirring constantly until thickened enough to coat the back of a spoon.
Transfer to a heatproof bowl and set aside to cool. When cooled, set aside in the fridge until needed.
Turn the dough out onto a lightly floured surface and knock back, lightly kneading until smooth. Roll into a sausage and cut into 12 equal pieces. Shape into balls and place on a baking tray and cover with oiled cling film. Leave to prove for about 30 minutes.
Preheat the oven to 200C/fan oven 180C/400F/Gas 6.
When risen, press down an egg-sized indentation into each bun and fill with the custard. Brush the bun with beaten egg.
Bake for 20 minutes, or until golden-brown. Transfer to a wire rack to cool.
Combine the icing sugar with one teaspoon of water to make a glaze.
Brush the glaze onto the buns around the custard centre. Dip the glazed part of each bun into the coconut.
Finish by placing a little jam on top of the custard and serve.