Add ¼ cup of erythritol to a food processor and blend on a high-speed setting until powdered – about 4/5 minutes. Set aside.
Whilst the erythritol is blending, add the diced rhubarb to a small pan with the water and remaining erythritol. Bring the mixture to a boil and stir to combine, then reduce to a simmer for 3-5 minutes, until the rhubarb is soft.
Strain the water from the rhubarb and mash the fruit lightly with a fork. Set aside to cool.
Add the powdered erythritol, whipping cream, cinnamon, ginger and vanilla to the bowl of a stand mixer.
Using a whisk attachment, whisk on a high-speed setting until the cream begins to thicken- this may take a couple of minutes. As it starts to thicken, reduce the speed and continue to whisk, forming thick and fluffy peaks.
Divide the mashed rhubarb mixture in half and carefully fold one half through the whipped cream.
With the remaining half, add a heaped teaspoon to the base of a ramekin or serving glass. Divide the cream and rhubarb mixture evenly between the 4 serving vessels and then top each with the remaining rhubarb. Serve immediately or chill a little before serving.