Preheat oven to 180 C / Gas 4. Grease a 20x30cm or similar sized baking tin.
In a medium bowl, mix together the butter, dark brown soft sugar and egg until smooth. Stir in apple sauce. Combine the flour, bicarbonate of soda and pumpkin pie spice; stir into the apple sauce mixture until well blended. Spread evenly into the prepared tin.
Bake for 25 minutes in the preheated oven or until edges are golden. Cool in the tin over a wire rack.
In a small bowl, mix together the icing sugar and margarine. Stir in vanilla and milk until smooth. Spread over cooled bars before cutting into squares.