Preheat oven to 180 degrees C. Grease 12 muffin cups or line with paper muffin cases.
In a large bowl cream together margarine and sugar. Beat in egg. Add banana and blend until it is completely mixed in then add flour, baking powder and bicarbonate of soda. Mix batter with spoon. Stir in soy milk then spoon into muffin tray. Spoon batter into prepared muffin cups.
Bake in preheated oven for 15 to 20 minutes until a toothpick inserted into centre of muffin comes out clean.