Combine custard powder and sugar in a saucepan, gradually whisk in the milk. Stir over medium heat until the custard has thickened, stir in butter and vanilla. Cover the surface of the custard with plastic wrap and cool.
For the cake, beat butter and half of the sugar until light and creamy, add eggs one at a time and continue mixing until well combined. Fold in flour and custard powder.
Spread half the cake mixture over the bottom of a greased and lined 23cm spring form tin, layer with custard and dot remaining cake mixture on top, gently spreading with a spatula.
Arrange rhubarb over the top of the cake and sprinkle with remaining sugar, cinnamon and lemon rind. Cover with foil and bake at 180°C for 35 minutes.
Remove foil then cook a further 40 minutes or until firm. Cool slightly in the pan, before serving.
Serve warm or at room temperature with cream or ice cream.