Cream sugar and margarine; add egg, then milk. Sift together flour, baking powder and salt; add to above mixture. Stir in cherries. Bake in greased and floured 8x8 inch pan at 350ºF. for 1 hour. If canned cherries are used, drain and use juice to make sauce for puddings, ice cream, etc. Cook juice; add cornstarch mixed with small amount of water; cool. Should be used within a week.