Whisk 250g butter until soft and fluffy. Add the icing sugar, a little at a time, whisking as you go until completely combined. Add the vanilla extract and whisk again.
Split the buttercream into two portions - one for filling and one for covering the cake. Split each semi-circle cake and fill with the buttercream.
Using a round cookie cutter, mark out a circle in the top of one of the cakes.
Carefully remove the small circle of cake to form the concave base of the laser gun.
Stick the two halves of the cake together with buttercream, with the circle cut-out on the top, then place on a cake stand or board.
Cover the whole cake with the remaining buttercream. I used a pallet knife to help spread it around evenly.
Roll out approx 350g of the fondant icing and use to cover the cake. If it breaks or cracks in places, don't worry too much - you can cover any blemishes in the next step! Trim off any excess fondant and remove, then tuck the edges under the bottom of the cake.
Roll out the remaining fondant icing. Using a large round cookie cutter, slightly larger than the one you used to cut out the laser gun base, cut out a circle of fondant. Decorate it by gently scoring with smaller round cookie cutters and a knife tip.
Cut a tiny circle from the middle, then push in the little cup light holder. Place the decorated circle of fondant, complete with light holder, in the concave round hole on the top of the cake.
Cut rectangles from the remaining fondant and decorate by scoring lines and little circles, using a tiny circle cutter or the end of a drinking straw.
Decorate the whole cake with the rectangles, making sure you cover up any cracks in the fondant icing base! If the icing is starting to dry out, you can stick them in place by brushing a little water on the backs. As you can see in the photos, I used the circle cutter to cut curved pieces from the rectangles around the laser gun area.
Spray the whole cake with the silver edible lustre spray. This step really made the cake shine, so if possible, don't skip it!
Press sugar pearls all over the cake to look like the lights of the death star. I took care to place them in the circles I had already marked on the cake.
Finish the cake with the cake light - turn it on first then pop it into the holder in the centre of the laser gun that you prepared earlier.