Brezeln

Spread the bicarbonate of soda on a lined tray and bake it at 120°C for 1 hour. This turns it into sodium carbonate, which is strong enough to give a real pretzel crust.

Mix the yeast and sugar into the lukewarm water, then work in the flour, the salt and the softened butter and knead for 10 minutes into a firm, smooth dough.

Divide into 8 and rest for 15 minutes.

Roll each into a rope 60 cm long, thick in the middle and thin at the ends.

Make a U, cross the ends twice, and press them onto the thick base to form the pretzel shape.

Chill uncovered for 30 minutes so the surface dries — dry pretzels take the dip evenly. Meanwhile, heat the oven to 220°C.

Dissolve the baked soda in 1.5 litres of water in a stainless-steel or glass bowl, wearing gloves; the solution corrodes aluminium, so keep it away from aluminium pans and trays. Dip each pretzel for 20 seconds, lift out with a slotted spoon and put on a lined tray.

Slash the thick part, sprinkle with coarse sea salt, pretzel salt if you have it, and bake at 220°C for 15 minutes until deep brown.