Put the flour and salt in the bowl of a stand mixer with the dough hook fitted. Rub in 10g of the butter. Mix the yeast with the water, then add to the flour and mix to form a dough. Knead for 6-8 minutes until it becomes elastic, then chill for 1 hour. Take the remaining butter out of the fridge and shape it into a rectangle measuring 10x6cm.
Roll the dough into a rectangle large enough to cover the butter on a lightly floured surface. Place the butter in the centre and fold over the sides of the dough to encase it. Roll the dough into a rectangle, fold one third of the dough up from the bottom like a letter, fold the top down to cover the bottom third of dough.
Turn the dough clockwise 45 degrees, roll it into a rectangle and repeat the same fold and turn. Cover the dough in cling-film and leave it to rest in the fridge for at least 1 hour.
Preheat the oven to 180C (350F/Gas 4) and grease 12 pastry rings and place them on a non-stick mat on a baking tray, or grease a muffin tin. Sprinkle half the sugar on the work surface and roll out the dough, sprinkle the remaining sugar on top and make another turn, folding the dough like a letter.
The dough should take all of the sugar, roll it so that it is all included and the dough makes a square that is 4mm thick. Cut the dough into 12 9cm squares and work quickly to keep the kouign-amann cool.
Make a sugar mound on the work surface and place a square of the dough on top. Fold in all four corners and press them into the centre. Take the new four corners and press them into the centre, so you have almost what looks like a circle of dough now.
Place the sugared dough into the prepared pastry ring and repeat with the remaining 11 dough squares, bake for 25-30 minutes until golden. Leave to cool slightly, then run a knife or spatula around the edge of the rings or tin and release the kouign-amann. Don't leave them too long or the sugar will set hard and you'll never free them in one piece from the mould.