Place the flour, sugar, cardamom and salt in the bowl of a free standing mixer with a dough hook attached and then add the cooled tanghzong.
Sprinkle the yeast into the milk in a bowl and stir to dissolve. (If using dried just add it to the flour). Pour the milk into the flour and mix on a slow speed. Now add the egg. Finally add the butter, mix until you have a smooth, silky, elastic dough, approximately 5-6 minutes. You may need to add an extra tablespoon of flour if the dough sticks to the side of the mixing bowl. Place in a plastic box with a lid and seal. Pop in the fridge overnight to prove. (up to 20 hours).
When you're ready to bake the next day knock back the dough and divide into 12 even pieces. Roll each piece in turn on a lightly floured work surface using your cupped hand until it forms a smooth bun. Place the buns on a baking sheet lined with greaseproof paper. Leave to prove covered for a further 45-60 minutes depending on the room temperature (the dough is colder so it will take longer) or to speed the proving up you can place the buns in the oven, SWITCHED OFF with a bowl of freshly boiled water, don’t cover them. They will take 30-40 minutes to prove.
Meanwhile pre-heat the oven 200°C fan/Gas 7.
Brush each bun with egg wash and then bake in the middle of the oven for 6-8 minutes or until golden brown. Leave to cool on a wire rack.
Once the buns have cooled, they need to be hollowed out. You can either cut a triangle or literally slice the top off, then scoop out 1/3 of the insides and then crumble. Weigh out 200g of the almond paste. (Freeze the remainder for another time). Grate the almond paste into a bowl and then add the milk and the bun crumbs to make a smoothish paste with the icing sugar. It should be a nice soft pipe-able consistency, you may have to add more milk than stated.
Add 1-2 teaspoons of icing sugar and the vanilla extract to the cream and whip until the cream just holds its form.
Fill the buns with the almond filling using a spoon or a piping bag. Pipe or spoon the cream on top and then put the tops back on. Dust with icing sugar and enjoy.