Preheat the oven to 350°F (180°C). Grease and flour the sides of two 8-inch cake pans, and line the bases with discs of wax paper.
Cream the butter until soft, then gradually add the sugar, and beat until light and fluffy. Add the eggs one by one, beating well all the time. Sift the flour and baking powder, and stir in gently, then stir in the milk until just combined.
Divide the mixture between the two pans, hollowing it slightly in the center, so that it will be flat on top when cooked. Bake for 20–25 minutes, or until the center of the cake springs back when you push it gently. Turn out onto a wire rack and allow it to cool. (Set the cake that will become the top layer on its base so that the top isn’t marked by the cooling rack.)
Meanwhile, place the sliced rhubarb, sugar and water in a saucepan, cover and cook over a gentle heat for about 10 minutes, until the rhubarb is soft. Take off the lid and boil while stirring, until the mixture is thick. Pour into a bowl and allow to cool.
Whip the cream until it forms soft peaks, then fold in the rhubarb. Sandwich the cakes with the rhubarb cream and sprinkle with sifted confectioners’ or superfine sugar.