Preheat oven to 180 C / Gas 4. Grease a 20x30cm or similar sized baking tin.
In a medium bowl, cream together margarine and dark brown soft sugar until smooth. Blend in the apple sauce and egg.
Combine the flour, bicarbonate of soda, salt and pumpkin pie spice, stir into the apple sauce mixture. Fold in raisins.
Spread the batter evenly in to the prepared tin. Bake for 20 to 25 minutes in the preheated oven, until firm. Let cool.
Meanwhile, make the icing. Heat butter in a small saucepan until melted and golden brown. Mix in icing sugar and vanilla. Gradually stir in the milk until icing is smooth and spreadable. Spread over cooled cake before cutting into squares.