Peel your spuds and chop them into even pieces. I think about 2-inch cubes works because the greater the surface area, the more potential for crispy corners..
Put them in cold water with 2 tsp of salt and 2 tsp sugar. The sugar and salt work together to bring out the flavour. You could swap the sugar for honey.
Bring to the boil and cook for about 10 minutes. You want the outer edge to start to soften, but for the potato to still be hard.
Drain the water off and put the potatoes back in the pan. Add 100g butter and a sprinkle of rock salt. Pop the lid back on and shake the potatoes until the edges have fluffed up and each potato is coated in the butter. It's important not to overcook the potatoes, so that they don't turn to mush at this point.
Place on a parchment lined tray, making sure the potatoes aren't touching each other. This will allow for maximum crispness. Allow the potatoes to cool before putting them in the oven. This will set the butter on each potato, but will also get rid of the steam, which stops the potatoes from crisping.
Roast in the oven for 1 hour at 170⁰. This temperature may seem quite low, but the smoking point of butter is around 175⁰, so we want to keep it below that.