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Heat the oven to 180c/160 fan/gas mark 4. Grease and bottom line two deep 20cm spring form cake tins. Using the microwave on high, melt the butter with the chocolate in 30 second bursts until glossy and smooth. Alternatively melt in a bowl over a pan of simmering water. Leave to cool slightly.
Combine the sugar and flour in a large bowl, getting rid of any lumps with your fingertips, then stir in the bicarb and salt. In a separate jug beat the eggs with the sour cream then pour into the dry ingredients and whisk until well combined. Scrape in the cooled chocolate mixture and beat until you have a smooth chocolatey batter. Stir in the hot coffee then divide the mixture between the tins and bake for 40-45mins until a skewer inserted in the centre comes out clean. Leave to cool in the tins for 10 mins then transfer to a wire rack to cool completely.
While the cakes are cooling make the buttercream. Using a food mixer or an electric whisk beat the butter with the icing sugar, vanilla and a pinch of salt for 10 mins until very light and fluffy, then whisk in the milk. Set aside.
Slice each of the cooled cakes into two. Break the chocolate bars into small pieces and stir the sea salt through the caramel. Sandwich the cakes together using a 1/3 of the caramel and chocolate bar pieces between each layer, leaving the top layer free for icing.
Crumb coat the cake completely in a layer of white buttercream then pop in the fridge to set. Meanwhile divide the remaining butter cream between 4 bowls, and colour each with the black, blue, green and purple food colouring.
Using a palette knife dot the cake at random with a little of each coloured buttercream then using the cake scraper go around the sides to blend the colours together and smooth the cake. Keep layering different colours of buttercream until you have your desired galaxy appearance.
Dot areas with the navy and silver lustre dust so that you create a moon beam effect around the cake. Using the paintbrush and white pearlescent paint, draw clusters of stars and dot in silver balls, being as random or as coordinated as you and your galaxy like.