In a large bowl, cream together butter and sugar until smooth. Beat in egg and vanilla until well blended. Combine flour, salt and baking powder; stir into the creamed mixture alternately with the milk. Spread evenly in a 9x9 inch baking pan.
For the topping, stir together all ingredients and mix with a pastry blender until the mixture is crumbly. Sprinkle over the top of the cake.
Bake for 25 minutes in the preheated oven, until a knife inserted into the center comes out clean.
This is a slight modification from the original recipe - more butter was added to the cake mixture, and the topping was doubled. The original holds fond memories for many, but we think the changes make a slight improvement!