Crack open 10 of the cardamom pods with a pestle and mortar, and tip into a saucepan. Add the milk and warm until steaming but not boiling. Add 50g butter (leave the remaining butter at room temperature to soften) and set aside to cool until lukewarm, swirling the pan from time to time to encourage the butter to melt.
Put the flour, 75g of the sugar, the yeast, cinnamon and 1 /2 tsp salt into a large bowl, or the bowl of a freestanding mixer. Mix until well combined. When the milk has cooled, strain it through a sieve into the flour, discarding the cardamom. Using a wooden spoon or dough hook, mix to form a soft dough. Tip onto a work surface and knead for 10 mins, or run the freestanding mixer for 5 mins, until the dough is smooth and stretchy. Clean the bowl, lightly grease with oil, then return the dough to the bowl and turn it over until well coated in oil. Cover the bowl with a tea towel or cling film and leave to rise for 2 hrs or until doubled in size (you could prove it in the fridge overnight).
Crack the remaining cardamom pods using a pestle and mortar. Prise them open and tip the seeds back into the mortar, discarding the pods. Crush the seeds to a powder, then combine with 150g sugar. In a bowl, mix the remaining butter with all but 2 tbsp of the cardamom sugar.
Line two baking trays with parchment. Punch the dough down to knock out the air, then roll to a rectangle roughly 35 x 45cm, with the longer edge facing you. Spread the cardamom butter over the surface, right to the edges. Fold the top third down to the middle and the bottom third up, like an envelope, so you have three layers of dough. Score, then cut into 12 equal strips, measuring about 3.5 x 11cm each. Cut each strip down the centre, leaving it attached at the top. Twist each strip away from the centre two or three times, then tie the dough in a knot and tuck the ends underneath the bun. Put each on the tray when done.
Cover both trays with a sheet of lightly oiled cling film and leave somewhere warm to rise for 30 mins - 1 hr, or until almost doubled in size. Heat oven to 190C/170C fan/gas 5.
Uncover the buns and brush all over with the beaten egg, then sprinkle with the pearl sugar. Bake for 20-25 mins until golden brown – swap the trays halfway through if they’re browning unevenly.
Tip the remaining cardamom sugar into a pan and add 50ml water. Bring to a boil, then remove from the heat and set aside to cool, swirling to dissolve the sugar. Brush the syrup over the buns two or three times as they cool, leave them to soak for 20 mins before eating. Will keep for 2 days in a sealed container, or freeze for 2 months. Defrost at room temperature and reheat for 5 mins in the oven before eating.