Cake: In a large bowl cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Sift together the flour, baking powder and salt. Beat in the flour mixture alternately with the milk, mixing just until incorporated.
Topping: Lay out the slices of tinned peach and pear across the base of a square cake tin. Sprinkle with walnuts and brown sugar. Tip the cake mix over the top.
Bake in the preheated oven for 40 minutes, or until a toothpick inserted into the centre of the cake comes out clean. As soon as it is comes out of the oven tip the cake onto a serving plate.