Grind the cardamom seeds into powder with a mortar and pestle.
In a small bowl, combine the butter, brown sugar, granulated sugar, vanilla, and cardamom. If the butter isn't soft enough you can pop it in the microwave for a few seconds first.
Set this filling aside for later, when you'll spread it across the finished dough.
Dough
In a large mixing bowl, dissolve the fresh or instant yeast in the milk.
Grind the cardamom seeds into a powder with a mortar and pestle. Add the sugar, cardamom, and cinnamon to the milk and stir until dissolved.
Add about half of the flour to the milk/yeast mixture and mix until smooth. Slowly add the rest of the flour and the salt, until you can knead the dough without it sticking (you may need less flour). Knead the dough on a floured surface for about 5 minutes, and then add the butter and continue kneading for another 5 minutes. You may need to add another handful of flour if the dough is too greasy with the butter.
Roll out the dough into a large square.
Assembly
Take the cardamom filling mixture you set aside earlier and spread it evenly across the square of dough. Fold the dough in half. Roll the folded dough out again, being careful to smooth out any air pockets.
Use a pastry or pizza wheel to cut about 20 thin strips out of the dough. Twist each strip and tie into a knot (see video).
Line baking sheets with baking paper and place each bun on the baking sheets with plenty of space in between. Cover with plastic wrap or a cloth and let rise for about 2 hours, or until the dough completely bounces back when pressed lightly with your finger.
Preheat oven to 200°C (390°F). Whisk the egg with a splash of water and gently brush onto the top of each bun.
Bake buns for 12 - 15 minutes in the middle of the oven, until the tops are golden brown.
Decoration
Melt the butter in a small bowl.
Grind the cardamom seeds in a mortar and pestle. Combine with the sugar in a shallow bowl.
Brush each cardamom bun with melted butter and dip in the cardamom sugar mixture.