Preheat the oven to 180°C/160°C fan/Gas Mark 4. Grease and line the base of three 20cm tins with baking paper. Dust the greased edges of the tins with a little flour, tapping out any excess.
In a large bowl, cream the butter and sugar together with an electric mixer until light and fluffy. Mix in the eggs, one at a time, until fully incorporated. Mix in half of the flour, then half of the milk, mixing until no floury or milky patches remain, before adding the remaining flour and milk. Mixing on a low speed, add the vanilla extract and pistachios until incorporated.
Carefully fold the blackberries into the batter by hand, to avoid the mixture turning an unappetising blue-grey colour from the juices they can release.
Divide the batter evenly between the lined tins. Bake for 20–25 minutes until the sponges spring back to the touch and an inserted skewer comes out clean.
Leave to cool in the tins for 10 minutes before gently turning the sponges out onto a wire rack to cool completely.
To make the buttercream, melt the white chocolate by placing it in a bowl set over boiling water (don’t let the base of the bowl touch the water). Alternatively, gently melt it in the microwave, being careful to stop and stir it every 30 seconds to avoid burning it.
In a large bowl, beat the butter for 1–2 minutes, then gradually add the icing sugar, a spoonful at a time, mixing it in before adding the next. Once all of the icing sugar is incorporated, mix in the vanilla and melted white chocolate.
When the sponges are completely cool, trim the domed tops off with a bread knife to make them flat.
Place a cake board or serving plate on a turntable or work surface and smear a little buttercream on the top. This buttercream will act as ‘glue’ and stop the cake from sliding around. Peel off the baking paper from the sponge layers and place the first sponge layer, cut-side up, on the cake board or plate. Spread a quarter of the buttercream on the top of the sponge, pushing it just over the edge.
Place a second sponge layer on top of the buttercream, cut-side down. Spread another quarter of the buttercream on the top of this layer, pushing it just over the edge.
Place the third sponge layer on top, cut-side down. Spread another quarter of the buttercream on the top, pushing it just over the edge.
Spread the final quarter of the buttercream around the sides of the cake, until the whole cake is covered in an even layer of buttercream and any gaps are filled. Place a bench scraper at a 90-degree angle against the side of the cake and turn the turntable (or your plate) to smooth the sides flat and create a patchy or ‘naked’ look to the icing on the sides of the cake. (Don’t worry if you push some buttercream onto the top of the cake).
To finish the cake with a smooth, 90-degree angle on the top, use a palette knife to neatly drag the top edge of buttercream into the middle of the cake.
Decorate the top of the cake with a Luminary crescent moon shape using all of the decoration ingredients. First, sprinkle the finely chopped pistachios into a moon shape, tapering off the ends – this will be your guide for the rest of the toppings. Place the blackberries in the shape, with the whole ones at the widest section of the moon and the smaller pieces towards the ends. Repeat with the whole pistachios and white chocolate buttons. Finish by placing small sprigs of rosemary and mint leaves in among the fruit and use more chocolate buttons and pomegranate seeds to fill in any gaps.