Heat corn syrup and 1 tablespoon margarine in skillet over medium heat until melted. Stir in brown sugar and cook until bubbly. Pour into ungreased 9-inch round baking pan. Spread pineapple on top.
Combine flour, baking powder, and salt. Set aside.
Beat remaining margarine until fluffy; then beat in sugar, egg, and vanilla. Add flour mixture to margarine mixture. Pour over pineapple.
Bake at 350° for about 40-45 minutes. Cool in pan and then invert cake onto serving platter.
Serve warm or cool, and top with whipped topping if desired.