Measure out two cups of water and put it in the saucepan you plan to use.
Put a ruler, skewer or chopstick into the water in the saucepan and mark the level on it.
The posset will need to reduce to that line.
Recipe adapted from Cook's Illustrated, March & April 2016 Magazine
In a medium (deep) saucepan, combine the cream, Splenda and lemon zest.
Bring to a boil over medium heat and cook, whisking frequently, to dissolve the sugar. KEEP AN EYE OUT! It will boil over quickly! (If it gets close to the top, remove the pan briefly from the heat to slow the boil.)
Cook until the mixture reduces to exactly two cups.
Remove from the heat and whisk in the lemon juice.
Set the saucepan aside to cool until a skin forms on top (approximately 20 minutes).
Strain the posset through a fine mesh strainer into a bowl.
Pour into 4 small bowls.
Refrigerate, uncovered, until set, about 3 hours.
Put the raspberries in a small food processor and whiz until broken down.
Spoon gently onto the top of the posset and decorate with raspberries, then put back in the fridge until needed.