1. Add the sugar and the cold water to a medium frying pan and heat very gently, stirring only occasionally until the sugar has completely dissolved – about 5 minutes.  Increase the heat and boil for 3-4 minutes without stirring until the syrup thickens and then begins to turn golden around the edges.  Watch very closely as the syrup begins to colour and tilt the pan gently from time to time to even this colour out. Cook for a minute or two more until golden all over.
2. Take the pan off the heat and add the boiling water, remembering to stand well back as the syrup will spit.  As soon as the bubbles begin to subside, tilt the pan to mix the syrup with the extra water.
3. Add the blueberries and cook for 1 minute or until the juices just begin to run from the blueberries, then leave to cool.  Stir the crème fraiche and Greek yogurt together in a bowl and chill until needed.
4. When ready to serve, add alternate spoonfuls of the yogurt mixture and the caramelized blueberries to 4 serving glasses. Serve like this or for a marbled effect, run a small knife or handle of a teaspoon through each dessert to swirl together.

Cook’s tip

Resist the urge to stir the syrup as it boils or you may find that it will crystallise into a solid white mass. If this happens, clean the pan and begin again.