Grease deep 23cm round cake pan, cover base with paper; grease paper. Cream butter, essence and sugar in small bowl with electric mixer until light and fluffy; beat in eggs 1 at a time. Transfer mixture to large bowl, stir in sifted flour, nuts and milk.

Spread half the cake mixture into prepared pan. Drop teaspoons of rhubarb filling and custard alternately on to cake mixture. Top with remaining cake mixture, spread gently. Bake in moderate oven about 1 hour or until browned and cooked through. Stand cake in pan 5 mins before turning onto wire rack. Serve cake warm or cold.

~=Rhubarb Filling=~ Combine rhubarb, sugar and water in pan, simmer, covered, until rhubarb is soft. Simmer, stirring, until thick; cool.

~=Custard=~ Combine milk, sugar and essence in pan, stir in blended custard powder and extra milk. Stir over heat until mixture boils and thickens. Pour into bowl, cover surface with plastic wrap; cool.