1. Gently heat the milk in a saucepanuntil lukewarm (about 38˚C). Tip intoa large bowl, then add 1 tsp sugar and the yeast, whisking until dissolved. Cover with cling film and leave to activate for about 15 minutes.
2. Mix in the remaining 35g sugar, the melted butter and ½ the beaten egg (reserve the rest for glazing), followed by the flour, cardamom and salt until you have a smooth and slightly sticky dough.
3. Knead the dough on a lightly oiled work surface for 10 minutes (or 5 minutes in a freestanding mixer fitted with a dough hook). Transfer to a bowl, cover with cling film and leave to rise in a warm place until doubled in size, about 30-40 minutes.
4. Turn out the dough onto a lightly floured surface and knead for 2-3 minutes. Use a rolling pin to roll it out to a rectangle about 40cm x 50cm and 0.5cm thick.
5. For the filling, beat the butter, vanilla seeds or extract and sugar in a bowl until smooth. Use a spatula or palette knife to spread it evenly over the rolled dough. Grate the marzipan over the filling (freeze the marzipan for 5 minutes to make this easier), then scatter over the blueberries.
6. Starting from one of the long ends, carefully roll up the dough into a long sausage. Take your time and squeeze the roll gently as you go to ensure the blueberries stick into the dough andfilling. When done, pinch the long edgeto seal and secure the dough.
7. Using a sharp, serrated knife, cut the dough roll into 16 even-sized swirls. Place on a parchment-lined baking tray, spaced about 3-4cm apart. Leave to prove under a tea towel for another 30 minutes.
8. Preheat the oven to 200˚C, gas mark 6. Brush each bun lightly with the remaining beaten egg and bake for around 15-20 minutes, until cooked through and a rich brown colour. Watch carefully as the buns can burn easily; move them around the oven if they are not baking evenly.
9. Remove from the oven and immediately cover the buns with a slightly damp, clean tea towel for a few minutes to prevent them from drying out. Meanwhile, in a small pan, dissolve the extra 2 tbsp caster sugar with 1 tbsp water and boil for 1-2 minutes, until syrupy. Brush over the warm buns and scatter over the toasted flaked almonds before serving.