1.Preheat oven to 160°C/300°F. Grease oven trays; line with baking paper.
2.Make rhubarb custard. Blend custard powder and sugar with milk in small saucepan; stir over heat until mixture boils and thickens. Remove from heat, stir in rhubarb. Cover surface of custard with plastic wrap; refrigerate until cold.
3.Beat butter, extract and sifted icing sugar in small bowl with electric mixer until light and fluffy. Stir in sifted custard powder and flour, in two batches.
4.With floured hands, roll rounded teaspoons of mixture into balls. Place about 5cm apart on trays; flatten slightly with a floured fork.
5.Bake biscuits about 15 minutes. Stand on trays 5 minutes, before transferring to wire racks to cool.
6.Sandwich biscuits with a little rhubarb custard. Dust with extra sifted icing sugar.
7.Make stewed rhubarb combine chopped rhubarb, sugar and water over a low heat until rhubarb is pulpy. Drain and cool before using.