Spread sugar evenly in a medium saucepan with a thick base.
Place the saucepan on the stove and allow the sugar to caramelize over a medium heat without stirring it first. As soon as the bottom layer of sugar has caramelized, stir it carefully with a silicone spatula until the sugar has completely caramelized to a dark brown and there are no lumps in the caramel. Make sure that the caramel does not burn; if necessary, you can reduce the heat or remove the saucepan from the stove briefly.
Once the sugar has caramelized, immediately switch the heat to low, carefully pour boiling water into the caramel in small portions while stirring (very hot and can splash!) and stir until a homogeneous caramel liquid without sugar lumps is formed.
Keeping the heat on low, add chopped butter to the caramel liquid in the saucepan and stir until the butter has melted.
Continue to add baking soda, cinnamon, ginger, cloves, nutmeg and salt to the caramel mixture over a low heat and stir until the mixture foams very well and increases significantly in volume (this happens quite quickly).
Remove the caramel mixture from the heat and leave to cool to lukewarm.
Stir egg yolk into the caramel mixture.
Add flour in portions and knead into a firm, slightly sticky dough.
Wrap the dough in an airtight container and leave it to mature in the fridge for at least 8 hours or overnight. It will become very firm during this time.
Leave the dough at room temperature to warm up a little and soften. Roll it out between two sheets of baking paper to a thickness of approx. 5 mm and cut out figures of your choice using different cookie cutters.
Spread the gingerbread cookies well apart on a baking tray lined with baking paper (you can also leave them on the baking paper on which they were rolled out) and bake them in a preheated oven at 356 °F (180 °C) for approx. 7 - 10 minutes.
Leave the kozuli to cool on the baking tray. To make their surface very smooth, you can place a large cutting board on top of the gingerbread cookies for about 5 minutes while they are still hot.