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Heat the oven to 180C/160C fan/gas 4. Butter and line the base of three 18cm sandwich tins with baking parchment. Beat the butter, sugar and vanilla together in a large bowl with an electric whisk for 8-10 mins, or until pale and fluffy. Add the eggs, one at a time, beating well after each addition. Sift in the flour, baking powder and a pinch of fine salt, and fold in using a large metal spoon. Add just enough of the milk to create a dropping consistency.
Divide the batter between the prepared tins and smooth the tops with a spatula. Bake on the middle shelf of the oven for 25-30 mins or until golden and firm to the touch. Leave to cool slightly in the tins, then transfer to a wire rack to cool completely.
While the cake is cooling, make the buttercream icing. Beat together the butter, icing sugar and vanilla until pale and fluffy, beating in 4-6 tbsp hot water to loosen the mixture if necessary.
Stack the sponges, spreading the buttercream over each one using a palette knife, then use the palette knife to roughly coat the outside of the cakes with the remaining buttercream to create a crumb coat. Chill in the fridge for 30 mins.
Using more of the icing, spread it over the chilled cake using your palette knife so that there’s an even layer of buttercream across the whole cake and over the top. Chill in the fridge for 2 hrs until set, or ideally overnight.
To make the mirror glaze, soak the gelatine leaves in cold water for 5 mins until softened completely, then drain. Put the sugar, 200ml water and the condensed milk in a pan, whisk together, bring to the boil, then whisk in the drained gelatine leaves. Put the white chocolate in a bowl, pour over the sugar mix in the pan and leave for 1 min to melt. Whisk everything together, then divide into three bowls.
Using the gel colourings, colour each bowl a different shade. Put one of the colours that you would like to use as an accent colour on top to one side. Stir all of the colours occasionally leaving them to sit until they reach 30-32C so they are thick enough to glaze the cake without being too thin and running off the edges. Once at that temperature, pour the remaining two colours on top of each other and stir slightly to marble them together.
Place your chilled iced cake onto a cooling rack set in a deep tray (to collect the excess glaze). Pour over the mirror glaze which should be at 30-32C, allowing it to drip down the sides. Drizzle over the accent colour you’ve saved then, using a paint brush, flick over some gold lustre, if using. Leave it to set completely before slicing and serving. Will keep, in the fridge, for up to three days.