Put the rhubarb in a large saucepan, along with the sugar and orange liqueur. Let simmer for 15 minutes, covered, over medium heat, stirring often, until the rhubarb is very tender and the pieces have lost their shape.

Add the strawberries and simmer very gently, uncovered and stirring occasionally, until the strawberries are tender (about 10 minutes).
Remove from heat and add the vanilla. The mixture should have the consistency of a thick purée.
Put in a large bowl and let cool completely.

Beat the cream with an electric mixer until stiff peaks form when the beaters are lifted.
Add the cream to the rhubarb mixture, creating pink marbling.
Transfer to individual serving cups with a spoon and serve with dry biscuits.