Preheat your oven to 180°C (160°C fan) or Gas Mark 4. Grease and line the bases of two deep 20cm springform cake tins with parchment paper. In a microwave-safe bowl, melt the butter and dark chocolate together in 30-second bursts, stirring between each, until smooth and glossy. Alternatively, melt them gently over a bain-marie (a heatproof bowl set over a pan of simmering water). Once melted, set aside to cool slightly.

In a large mixing bowl, combine the soft brown sugar and self-raising flour, breaking up any lumps with your fingertips. Stir in the bicarbonate of soda and salt. In a separate jug, beat the eggs together with the sour cream, then pour this mixture into the dry ingredients and whisk until fully combined. Add the cooled chocolate mixture to the batter and whisk again until smooth and well incorporated. Stir in the hot coffee until the batter becomes glossy and pourable.

Divide the batter evenly between the prepared cake tins. Bake for 40–45 minutes, or until a skewer inserted into the centre comes out clean. Allow the cakes to cool in the tins for 10 minutes before transferring them to a wire rack to cool completely.

While the cakes are cooling, prepare the buttercream. Using a stand mixer or electric whisk, beat the softened butter with the icing sugar, vanilla bean paste, and a pinch of salt for about 10 minutes, until the mixture is extremely light and fluffy. Add the milk and whisk until fully incorporated. Set aside.

Once the cakes are cool, carefully slice each one in half horizontally, giving you four even layers. Break the mini Mars Bars into small chunks, then stir the sea salt into the caramel. To assemble the cake, place the first sponge layer on a serving plate or turntable. Spread over one-third of the caramel mixture, dot with Mars Bar pieces, and top with the next sponge. Repeat this process with the remaining layers, leaving the top layer plain for icing.

Apply a thin crumb coat of the white buttercream over the entire cake to seal in crumbs. Chill the cake in the fridge until the buttercream is firm to the touch.

Meanwhile, divide the remaining buttercream evenly between four bowls. Tint each batch with a different gel food colouring: black, blue, purple, and green. Using a palette knife, dab small amounts of each colour randomly over the cake. Then, using a cake scraper, gently smooth the sides and blend the colours to create a swirling galaxy effect. Keep layering and blending until you're happy with the look.

Once your galaxy base is complete, enhance the design by brushing on navy and silver lustre dust to create light effects and moonbeam-like glows. With a fine paintbrush dipped in edible metallic white paint, draw tiny stars and star clusters around the cake. Finish by pressing silver sugar pearl balls randomly over the surface to mimic stars or planets.