Preheat the oven to 180°C/Gas mark 4. Cream the butter and the 35g of sugar together in a bowl, add the egg, ginger and black treacle. Fold in the self raising flour. Take 100g of your leftover pineapple trimmings and boil with the water in a large pan. Pour the pineapple/water mix into a blender and blend until smooth, add the bicarbonate of soda and mix well.