Make the rhubarb compote:
In a saucepan over medium heat, combine chopped rhubarb, granulated sugar, lemon juice, and vanilla. Cook for 10–15 minutes, stirring occasionally, until the rhubarb breaks down and the mixture thickens slightly. Let it cool completely.
Prepare the whipped cream:
In a chilled bowl, beat the heavy cream, powdered sugar, and vanilla extract until soft peaks form. Set aside in the fridge.
Make the crumble topping:
Mix crushed biscuits, melted butter, and brown sugar in a small bowl. Toast lightly in a skillet over medium heat for extra crunch, if desired.
Assemble the fool:
Gently fold the cooled rhubarb compote into the whipped cream. Swirl lightly for a marbled effect—do not overmix.
Serve:
Layer the fool into dessert glasses with a spoonful of crumble in the middle or on top. Garnish with fresh mint and an extra dollop of cream if desired.