Heat oven to 180C/160C fan/gas 4, grease two 20cm cake tins with butter and line the bases with baking parchment. Add 225g butter and 225g sugar to a large bowl and beat with an electric whisk until the mixture is pale and light. In another bowl or a jug, mash 3 of the bananas with a fork. Add 3 eggs, 75g yogurt and 1 tbsp vanilla, and mix to combine. Pour the wet ingredients into the butter and sugar, and add 225g flour, 1 tsp baking powder and 3 tbsp custard powder. Whisk to combine until you have a smooth batter.
Divide the mixture between the tins, spreading it to the edges, then bake on the middle shelf for 25 mins – a skewer inserted to the centre should come out clean when the cakes are cooked. Cool in the tins for 10 mins, then turn out onto a wire rack. Clean the tins, grease with more butter and line the bases as before, then bake another batch of sponges wth the remaining ingredients. Can be made 2 days before icing; once cool, wrap well in cling film and leave in a cool place, or freeze for 2 weeks.
While the cakes cool, make the icing. In a small bowl, mix the custard powder, vanilla bean paste and milk until lump-free. In a large bowl, weigh the butter and icing sugar, then mash together with a spatula (this is to prevent covering the kitchen with icing sugar!). Once roughly combined, mix with an electric mixer until smooth. Scrape in the custard mixture and beat again.
When you’re ready to assemble, clear a space for the cake in the fridge. Place one of the sponges on a plate or cake stand, and use roughly a quarter of the custard icing to stack the other cakes on top, spreading the icing right to the edges between each sponge. Use a little more icing to thinly cover the entire cake, filling in any gaps between the cake layers and using a palette knife to smooth the surface. Try not to get any crumbs in the bowl of icing – this first coat is called the ‘crumb coating’ and will trap in any crumbs, so don’t worry if it looks a little messy at this stage. Chill the cake for 30 mins until the icing is firm.
Use the remaining icing to completely cover the cake. Hold your palette knife up against the side of the cake and swoop around it to give you a nice clean, flat surface. Smooth over the top as much as possible too. Wipe the plate or cake stand clean, then chill for another 30 mins.
Line a baking tray with baking parchment. Melt the white chocolate in a bowl suspended over a pan of gently simmering water, or in short bursts in the microwave. Pour roughly a quarter of the chocolate onto the baking tray and spread it to a rectangle, roughly 12 x 20cm. Sprinkle with hundreds & thousands and set aside to cool or chill until set. Mix the oil into the remaining chocolate in the bowl and add enough food colouring to dye the chocolate a vivid yellow colour.
Remove the cake from the fridge and pour the yellow chocolate icing over the top, using a teaspoon to encourage it to drizzle down the sides. Cut the set sheet of white chocolate into pointy shards and arrange them on top of the cake (you may have to use some of the sweets and biscuits to hold them in place). Stack the remaining sweets and biscuits on top of the cake and stick in the cake topper (if using). The cake will look its best for an hour or two after decorating, but leftovers will keep for up to 3 days in the fridge.