First cook 3 cakes. Use a 1litre bowl, a 6in square tin and an 8in square tin. Use a madeira cake recipe, with 3 eggs for the bowl, 3 eggs for the 6in and 5 eggs for the 8in.
When cool, trim all the crust off the cakes, to get a hemisphere from the bowl cake, and flat tops from both square cakes. Then stack the cakes, putting the 8in at the bottom, 6in in the middle, then the bowl cake at the top - hey presto, something that doesn't really look like a stormtrooper helmet.
Now you have to do some sculpting with a sharp knife to get the shape about right. The middle cake you can cut into more of a wedge shape, so the back of the helmet is lower than the front, and cut off the corners so it matches the shape of the bottom of the top cake. The top cake you can pretty much leave as it is. The bottom cake needs the most work. Put the middle cake nearer the back of the bottom cake, so there is only a small overlap at the back. Then the corners at the back need rounding off, as well a all the edges curving (including the edge between the cake and the cake board). At the front of the bottom cake, you need to shape it how it should look. Both sides come in a little, and shape to make the circle shapes at the front on both sides. Cut a small hole out of the middle of each circle. The middle at the front should curve down at either side of the middle cake. This cutting stage took me around an hour.
Now make up 500g of buttercream icing. First put some inbetween each cake, and then cover the whole cake up. Now from the big piece you cut off the 8in cake to make the top flat, cut a piece of cake that makes the nose section. Stick that onto the cake (the buttercream should be enough) and cover that with buttercream as well. About another hour to do all this.
Now get 1kg of white fondant icing. Roll out so that it will cover the whole cake and just about touch the cake board on all sides. Place over the cake. This took me ages as the icing I had needed a lot of work to get plyable, about another hour or so.
Now start arranging it so that it tucks into all the corners that you cut in the original cake. Near the bottom cut the excess away, then work the rest together so that it doesn't bunch up so much. I find a proper icing smoother (a bit like a plasters float, but much smaller) really good for getting all the finger marks off the icing. This took another hour to look really good.
Now get some black fondant, and roll out and cut to shape. Mix together white and black to get the grey sections, and use some of the rest of the excess to add the ear sections. Some edible glue from a proper cake shop is great here, together with a really small child's paintbrush to paint the side of the icing you want to stick on. About two hours for this part.