I used baking powder and bicarbonate in the cake sponges (and not in the cupcakes) as I felt it baked much better for the larger sponge size - I don't think the cupcakes need it though.
I used Tate & Lyle icing sugar which is a vegan icing sugar.
This cake is very soft - and will last for at least three days!
If your sponge turns out rubbery, it's most likely over mixed. I personally have never experienced this!