Cream butter and sugar. Beat in egg and milk. Lightly sift in flour, baking powder and salt, and fold in with a metal spoon.
Combine topping ingredients in a separate bowl.
Grease and base line a 23cm (9") square cake tin with foil. Spread half thin layer of the cake mix in the bottom. Spread with just over half the topping mix. Spoon over the rest of the cake mix and spread evenly with a fork. Scatter over the remaining topping and press down very lightly.