1. Mix the ferment in a bowl, cover with cling film and leave to rest in warm room temperature for 5 hours or overnight. It will bubble up slightly.
2. Add the rest of the water, flour, malt extract, salt and butter to the ferment and knead or mix in a standing mixer with a dough hook attachment until the dough is smooth, elastic and not sticky. Leave it in the bowl to rest for 15 minutes.
3. Turn the dough out onto a work surface and divide into 6 pieces. Roll each one into a long rope; if the dough keeps springing back, leave it for 5 minutes, then continue.
4. Shape each rope into a pretzel shape, as shown, place them on a tray and leave covered with cling film or a tea towel for 30 minutes in a warm place. Transfer to the fridge, uncovered, and chill overnight.
5. The next day bring a large pot with at least 2l of water to a boil. Stir in the baking soda and keep it on low simmer. Lightly grease a baking sheet (don’t line with parchment – the pretzels will stick). Preheat the oven to 220C (fan if available)/425F/gas 7.
6. Drop the chilled pretzels into the water, at most two at a time, for 30 seconds or until they float. Remove onto the greased sheet with a slotted spoon. When all has been cooked, sprinkle generously with coarse salt and make an incision in the thickest part of each pretzel with a sharp knife or baker’s lame.
7. Bake for 12-13 minutes in the upper part of the oven until deep brown.
8. Cool on a wire rack. They are the best on the day they're baked but will keep reasonably well overnight in a bread bag, going quite dry on the third day.