1.Preheat oven to 180°C/350°F. Grease oven tray.
2.Place rhubarb, the water and caster sugar in medium saucepan; bring to the boil. Reduce heat; simmer, uncovered, stirring occasionally, about 3 minutes or until rhubarb softens. Drain rhubarb mixture through sieve over medium bowl; reserve liquid. Spread rhubarb mixture onto metal tray; cover with foil, place in freezer.
3.Meanwhile, stir butter, brown sugar, syrup and ginger in same cleaned pan over low heat until butter has melted. Remove from heat; stir in flour.
4.Drop level teaspoons of mixture about 6cm (2½ inches) apart on tray. Bake about 7 minutes or until brandy snaps bubble and become golden-brown. Cool on trays 2 minutes; transfer to wire rack to cool completely.
5.Place cooled rhubarb mixture in small bowl; add yoghurt, pull skewer backwards and forwards through rhubarb mixture for marbled effect.
6.Sandwich 3 brandy snaps with a quarter of the rhubarb mixture; repeat with remaining brandy snaps and rhubarb mixture.
7.Place stacks on serving plates; drizzle with reserved rhubarb liquid.