Bring water to a boil. Add 1 teaspoon of sea salt and cook the potatoes for 8-10 minutes until al dente, peel and let cool. Peel and roughly chop garlic cloves. Halve the chilli pepper lengthways, remove the core and cut into strips. Pluck the rosemary needles from the stems.
Fry the potatoes vigorously in butter and olive oil. Reduce heat. Add the garlic and chilli and fry the potatoes until golden, turning. Add the rosemary, sprinkle the potatoes with brown sugar, season with salt, pepper and turn the sugar until it is liquid and brown. Deglaze with balsamic vinegar and serve immediately.