Line a baking tray with parchment paper and place it next to the stovetop. Put two trivets beneath the baking tray to protect the counter from the heat, making sure that the tray is levelled and secure in its place.
Bring the water to a boil: in a medium, heavy-bottomed saucepan pour the water (250 g; 8.8 oz) and bring it to a boil over high heat (100° C / 212° F / it bubbles up vigorously).
Add the sugar: remove the saucepan from the heat and add the sugar (300 g; 10 oz). Stir for 1 minute and 20 seconds; do not estimate it, time it. This is the time the sugar needs to dissolve; some sugar granules left are ok.
Caramelise the sugar: return the saucepan with the syrup over medium-high heat and cook until it is a deep brown caramel colour (195° C / 383° F if you use a thermometer). Do not stir while it cooks, but as the caramel darkens, do tilt the pan gently once or twice if you notice darker spots forming, to distribute the heat evenly.
Remove the saucepan from the heat and pour the caramel over the parchment paper, scraping with the rubber spatula caramel residues from the saucepan,
Let it cool down for approximately 30-40 minutes or until it doesn’t feel warm to the touch (this is at 27° C / 80° F if you use an infrared thermometer).
Note that the caramel is very sensitive to humidity, so from now on, take care that anything it comes into contact with is completely dry. Don’t leave it exposed to the kitchen’s humidity either; as soon as it comes to room temperature, either proceed with the recipe or put it in an airtight bag.
Break the caramel into pieces with your hands (dry, please) and put the pieces in a (completely dry) blender jug/food processor. Pulse to break the caramel to as fine as possible.
A blender creates a fine powder which dissolves easily. A food processor breaks the caramel into pieces, the size of a rice grain, which just take a little longer to dissolve.
Store the caramel sugar: immediately weigh the caramel sugar (200 g; 7.1 oz; all of it if measuring in cups) you need for the ice cream into a (completely dry) airtight container and close the lid. Proceed with the recipe, or keep it for up to one month. Any leftover caramel sugar can be stored in an airtight container and used to sprinkle over the ice cream or to flavour your coffee.