Preheat oven to 375°F or 190°C.
Cream the butter and sugar together, add the egg and beat well.
In a separate bowl, blend the flour, baking powder and salt together.
Mix the flour mixture into the creamed mixture, alternating with milk, until smooth.
Spread the batter into a 8" x 9" (20cm x 23 cm) baking dish.
For the topping, combine the brown sugar, flour, cinnamon, walnuts and melted butter.
Spread this mixture over the batter.
Bake for 30 to 35 minutes until golden brown and a toothpick inserted into the batter comes out clean.