Preheat the oven to 180°C.
Whip the double cream until soft peaks form, then whisk in icing sugar, yoghurt, liqueur, and vanilla.
Place rhubarb on a tray, sprinkle with sugar, add spices, and bake for 5 minutes until tender.
Cool the rhubarb slightly and fold it into the cream mixture.
Cream the butter and sugar for the biscuits until light and fluffy.
Stir in the egg, syrup, flour, baking powder, and both types of ginger until combined.
Roll into ball shapes and bake for 8-10 minutes until golden-brown.
Spoon the fool into glasses and serve with the biscuits.