Combine the chopped rhubarb, granulated sugar, and water in a saucepan. Cook over medium heat, stirring occasionally, until the rhubarb has softened and broken down, about 8-10 minutes. Mash gently with a fork.

Allow the rhubarb mixture to cool completely. For a smoother fool, you can press it through a sieve to remove any fibrous bits, or simply mash it well for a more rustic texture. Let it cool thoroughly.

In a separate chilled bowl, whip the cold double cream with the sifted icing sugar and vanilla extract until soft peaks form. Be careful not to over-whip.

Gently fold about two-thirds of the cooled rhubarb mixture into the whipped cream. You want to create swirls, not fully combine it.

Spoon the mixture into individual serving glasses or a single serving bowl.

Drizzle the remaining rhubarb mixture over the top and use a skewer or spoon to gently swirl it in for a marbled effect.

Chill for at least 30 minutes before serving. Garnish with a sprig of mint or a few slivered almonds if desired.