This cake looks incredibly impressive, but get the rhubarb right (a simple matter of not slicing them too thinly and making sure you bake them until cooked but still firm) and the rest is not very difficult at all. The sponges are deep, so you could slice each one in half horizontally and create more layers (dividing the buttercream proportionately), if you like.
Heat the oven to 180°C/160°C fan/Gas 4.
Make the two 20cm sponges. Tip the eggs and sugar into the bowl of a stand mixer fitted with a whisk. Whisk together for 4–5 minutes, until thick and pale and the whisk leaves a ribbon trail in the mixture when you lift it. With the whisk on slow, slowly pour in oil, then add the flour, baking powder, salt, milk and lemon zest and mix for 30 seconds. Remove the bowl from the stand mixer and gently fold in the lemon juice, limoncello and vanilla paste until you have a smooth batter.
Divide the batter equally between the two 20cm cake tins and bake for 50 minutes, until risen, golden brown and a skewer inserted into the centres comes out clean. Remove from the oven and leave to cool in the tins on a wire rack.
Repeat Step 2 for the 15cm sponges and the 10cm sponges. Divide the batter proportionately between the 4 lined cake tins, ensuring there is the same height of batter in each tin. Bake for 30–40 minutes, until risen, golden brown and a skewer inserted into the centres comes out clean. Remove from the oven and leave to cool in the tins on a wire rack.
While the sponges are cooling, make the syrup. Tip the lemon juice and sugar into a medium pan over a low heat and bring it to a simmer, stirring continuously, until the sugar has dissolved. Remove from the heat, pour it into a jug and leave to cool to room temperature.
While the sponges are still warm, prick them with the skewer and pour the syrup over. Leave to cool completely.
Make the thyme and raspberry jam. Place the raspberries, the lemon juice and zest and the thyme leaves in a deep-sided pan. Add the jam sugar and bring to the boil over a low heat until the sugar has melted. Increase the heat and boil for 4 minutes, until the temperature on the sugar thermometer reaches 105°C (setting point). Remove from the heat and carefully pour the jam into a shallow container. Leave to cool, then transfer to the fridge and chill to set.
Make the Swiss meringue buttercream. Place the egg whites and sugar in a large bowl set over a pan of gently simmering water and whisk with a balloon whisk until the sugar has dissolved and the temperature on the sugar thermometer reaches 71°C.
Transfer the warm mixture to the bowl of a stand mixer fitted with the whisk and whisk on high speed until the mixture is at room temperature and a thick meringue texture. Add the butter a few cubes at a time, whisking well between each addition to a smooth, silky buttercream. Slowly whisk in the raspberry powder, lemon zest and vanilla paste, then set aside until ready to use.
Make the roasted rhubarb. Heat the oven to 180°C/160°C fan/Gas 4. Trim the rhubarb and cut each stem in half to give about 16cm lengths. Slice the rhubarb on a mandolin into long strips about 2–3mm thick and lay the strips in a single layer on the lined baking sheets. (You will need to do this in batches.)
Mix the sugar, vanilla, food colouring and 1 tablespoon of water together in a bowl to a thick sand syrup. Using a pastry brush, brush each strip of rhubarb with the mixture, then roast it in the oven for 6 minutes, until cooked but still firm. Remove the rhubarb strips from the oven and slip them onto a cooling rack lined with baking paper. Don’t leave the rhubarb on the baking sheet as it will continue to cook and become too soggy to stand up. Continue slicing and roasting until you have about 70 strips of roasted rhubarb. Pat the rhubarb slices dry with kitchen paper.
Assemble the cake. First, remove the sponges from the tins and peel off the baking paper. Spoon a blob of buttercream onto a serving plate and place one of the 20cm sponges on top. Spread about one quarter of the buttercream in a thick layer over the cake, followed by half the jam, then place the remaining 20cm sponge on top, top side down.
Spoon a blob of buttercream onto the 15cm cake card and place one of the 15cm sponges on top. Spread a thick layer of the buttercream over the cake (to match the previous depth of buttercream), followed by just over a quarter of the jam, then place the remaining 15cm sponge on top, top side down.
Spoon a blob of buttercream onto the 10cm cake card and place one of the 10cm sponges on top. Spread a thick layer of the buttercream over the cake (to match the previous depths of buttercream), followed by the remaining jam, then place the remaining 10cm sponge on top, top side down.
Working with one sandwiched cake at a time, measure, cut and place 4 dowels in the 20cm cake, 3 dowels in the 15cm cake and 2 dowels in the 10cm cake.
Coat the sides of each of the cakes and the sides and top of the 10cm cake with the remaining buttercream, then smooth with a palette knife. Stick the strips of rhubarb to the sides of the three cakes, trimming to fit.
Stack the cakes one on top of each other, then arrange raspberries around the edge of each tier and on top of the cake. Wrap the ribbons (if using) around each tier, as in the photograph, and decorate with sprigs of thyme. Push the candles into the top (if using), to finish.