Pour the milk into a small saucepan and heat to 38°C, remove from the heat and add the fresh yeast, whisk to incorporate (if using fast action yeast add it to the flour). Place the flour, tangzhong, cardamom, sugar and salt in the bowl of a free standing mixer with the dough hook attached. Add the milk mixture and egg with the motor running. Mix until a dough forms on a slow speed. Once the dough has come together add the butter, increase the pace to medium/high and work the dough for another 5-8 minutes or until it feels shiny and smooth.
Place the dough in a bowl and cover with a tea towel and let it rest for 20-30 minutes to rest.
Lightly dust the work surface with flour. Roll the dough into a long sausage and then divide into 16 even sized pieces. Roll each piece with a cupped hand into a bun shape. Then take a rolling pan and roll out into a flat disc. Repeat with the other buns. Place onto a lined baking tray. Prove for 35-40 minutes or until well risen.
Preheat the oven to 200C fan.
Now take the creme patisserie and whisk with a balloon or a hand electric whisk, beat until smooth. Place the creme patisserie in a piping bag fitted with a 1cm plain nozzle.
Once the buns have risen, carefully insert the nozzle into the centre of the bun and pipe some of the creme pat into the bun then finish with a swirl on the top. Brush with beaten egg and bake for 8-10 minutes, or until golden brown.
Once the buns have cooled, bush with melted butter and roll them in caster sugar. Devour!