WHAT YOU NEED

For the Satsuma Soak
120g (4¼oz) caster (superfine) sugar
80ml (2½fl oz) water
4 satsumas or mandarins, skin on and cut into 5mm (¼ inch) slices

For the orange sponge
cooked satsuma slices (see above)
3 large eggs, room temperature
180g (6½oz) soft light brown sugar
110g (3¾oz) unsalted butter, melted and cooled
1 teaspoon vanilla bean paste
1 tablespoon orange zest
220g (7¾oz) plain (all-purpose) flour
1 teaspoon bicarbonate of soda (baking soda)
180ml (5¾fl oz) buttermilk

For the rainbow buttercream
600g (1lb 5oz) unsalted butter, room temperature
600g (1lb 5oz) condensed milk
1 tablespoon vanilla bean paste
⅛ teaspoon each of red, orange, yellow, green, blue and purple gel food colouring

For the satsuma soak
Combine the sugar and water in a saucepan over medium heat and, without stirring, bring it to the boil.
Add the satsuma slices and cook for 15 minutes on low heat.
Remove the satsumas from the pan. Reserve the cooking liquid as it will be used to soak the cakes once baked. The slices will be used to line the tin.

For the orange sponge
Preheat the oven to 160°C (315°F). Grease three 15cm (6 inch) round cake tins and line them with baking paper.
Place six slices of cooked satsuma at the bottom of each tin.
In a stand mixer with the whisk attachment, whisk the eggs and sugar on high speed until light and fluffy. While whisking, slowly pour the melted butter into the egg mixture and mix until combined. Add the vanilla bean paste and orange zest and mix to combine.
Sift one-quarter of the flour and one quarter of the bicarbonate of soda over the mixture and gently fold them in using a spatula.
Add one-quarter of the buttermilk and mix well to combine.
Repeat steps 4 and 5 until all the flour mixture and buttermilk are used.
Divide the reserved satsuma slices among the tins, then spoon the batter evenly on top, levelling the surface. Bake for 25 minutes until a toothpick inserted into the cakes comes out clean. Leave to cool in the tins.

For the rainbow buttercream
In a stand mixer with the paddle attachment, beat the butter until pale and creamy. While mixing, gradually add the condensed milk and mix to combine. Add the vanilla bean paste and mix until well combined.
Reserve two-thirds of the buttercream and leave it plain.
Divide the remaining buttercream evenly among six bowls and add a different food colouring to each. Mix until well combined and the colours are even.

To assemble
Prick the sponges with a toothpick, then drizzle the satsuma soak over them.
Place one sponge on a cake board or serving plate with the satsuma soaked side up. Use one-quarter of the plain buttercream to cover the top of the cake.
Repeat to place the second sponge on top and cover with another one quarter of the plain buttercream.
Top with the third sponge, then cover the top and side of the cake with the remaining plain buttercream, smoothing it out well with a cake scraper or palette knife.
Place the cake in the freezer for 30 minutes.
Remove from the freezer. Place a plastic stencil of a world map (with the map shape cut out of the plastic stencil) on the side of the cold cake.
To fill the template, use a small palette knife with a minimal amount of one of the purple buttercream to spread a stripe across the bottom of the world shape. Follow with a stripe of blue above the purple. Once all the colours have been applied in the order of the Pride flag (see photograph), carefully scrape off the excess buttercream with a cake scraper or spatula, then gently peel off the stencil.
Transfer the remaining coloured buttercreams to individual piping bags with star nozzles and pipe rosettes around the top of the cake.
Store in the fridge for up to 3 days. Serve at room temperature.