Preheat the oven to 150°C. Line a 22 cm round cake tin with baking paper.
In a large bowl, mix all the dry ingredients together well to combine.
Make 3 holes in the mixture: 1 large, 1 medium and 1 small hole. Into the large hole, add the melted margarine. Into the medium hole, add the vinegar. Into the small hole, add the vanilla. Pour the soy milk all over the top, then mix the whole lot together until there are no lumps.
Pour the batter out into the cake tin and bake for 18–20 minutes, or until a skewer inserted into the centre comes out clean. Remove from the oven and leave to cool completely in the tin. (Trying to take it out earlier will result in the cake breaking!)
Make the icing by mixing everything together in an electric mixer until smooth. Spread over the top of the cooled cake and decorate with berries, if desired.