Crumble the yeast into a kitchen mixer and heat the milk to 37 °C. Pour the milk over the yeast and stir to combine. Dice the butter and add all the ingredients, setting the mixer to run slowly for a few minutes. Next, increase the speed and knead the dough for a further 7 minutes.
Set the dough aside for 30 minutes under a kitchen towel. Prepare the fillings. Whisk all the ingredients for the cardamom filling and set aside. Now prepare the rhubarb filling. Rinse and chop the rhubarb into small pieces. Boil the rhubarb in the water with the sugar and vinegar in a saucepan for 3 minutes. Stir and set aside to cool.
Divide the dough into two, and roll each piece out into a rectangle on a floured surface. Spread a layer of cardamom filling over the whole rectangle. Cut the rectangle in half and then into small squares of about 8x8 cm.
Spoon rhubarb filling onto the centre and fold the edges of the dough over to meet at the centre. Place the parcels on greaseproof paper and cover with a tea towel. Leave to rise for about 90 minutes. Repeat for the other half of the dough.
Preheat oven to 210 °C.
Brush with beaten egg and bake the buns in the centre of the oven for approx. 10 minutes. Leave the buns to cool on a wire rack.
Mix the icing and drizzle over the buns. Serve.
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