Make the cake:

1. Position a rack in the centre of the oven and preheat to 350°F. Butter and flour three 7” cake pans and line the bottoms with parchment paper; if using a flour-based baking spray, apply just before filling.
2. Select the most colourful stalks of rhubarb, and using a vegetable peeler, peel one strip from each side of the stalks. In a medium bowl, toss the strips with 50 g or ¼ c of granulated sugar. Arrange half the rhubarb strips in a single layer in the bottom of one of the cake pans, trimming to fit. Turn the pan 90° and arrange the remaining strips over the first layer to form a tartan pattern.
3. Thinly slice the remaining rhubarb (you should have about 3 cups) and toss with 50g or ¼ c of granulated sugar. Spread 1 cup of the rhubarb across the bottom of each of the remaining cake pans. In a small saucepan, cook the final cup of rhubarb and sugar until tender (2 minutes). Zap with an immersion blender until smooth and set aside—you’ll use it for the frosting.
4. In a medium bowl, whisk together the flour, baking powder, baking soda, salt and 1 c of sugar.
5. In a stand mixer fitted with the whisk attachment, beat the egg whites on medium-low speed until foamy. Add the cream of tartar and continue beating at medium-high speed until soft peaks form. Gradually add the final 50 g or ¼ c of sugar and beat until glossy, firm peaks form (2–3 minutes). Transfer to a bowl.
6. In the bowl from the stand mixer (no need to wash), beat the buttermilk, melted butter, four egg yolks, vegetable oil and vanilla. Gradually add the flour mixture and mix until incorporated (15 seconds). Scrape down the bowl and beat on medium-low speed until smooth. Gently fold in the egg whites, 1/3 at a time, until no streaks remain.
7. Divide the batter evenly between the pans and smooth the tops with an offset spatula. Bake until the tops spring back when lightly touched and a toothpick inserted in the centre comes out clean (20–25 minutes). Let them cool on wire racks for 15 minutes before running a thin knife around the edges and removing them to cool completely. Discard the parchment paper.

Make the Swiss meringue buttercream:

1. Place the egg whites and sugar in a heatproof bowl (I use stainless steel) and whisk until combined. Place the bowl over a pan of simmering water, ensuring the bowl doesn’t touch the water. Continue to whisk until the mixture reaches 160°F or until it’s not grainy between your fingers.
2. Transfer the egg mixture to the bowl of a stand mixer, and beat with a whisk attachment until you get glossy, room temperature peaks. Tip: hold a couple of bags of frozen peas or corn against the outside of the bowl to bring the temperature down.
3. With the mixer running on medium-low, add small chunks of softened butter, one at a time. Take your time with this—let the butter incorporate fully before adding the next piece (5–10 minutes). Don’t panic if the mixture gets soupy. Just keep beating. You can’t overbeat it, and it will come together in the end. Tip: hold a bag of frozen peas or corn against the side of the bowl to bring the temperature down more quickly.
4. Beat in the reserved rhubarb purée.

Assemble the cake:

1. On a turntable or pedestal cake stand, place one of the rhubarb-bottom cake layers rhubarb-side up. Apply approximately ¾ c of frosting. Repeat with the second layer. Add the tartan ribbon layer to the top. Spread the sides of the cake with a thin layer of frosting. Transfer the cake to the fridge to firm up (15 minutes).
2. Fit a piping bag with a basketweave tip and fill with the remaining frosting. Pipe the frosting in vertical strips around the sides of the cake for a rustic wooden fence pattern.