Preheat the oven to 350 F. Butter a 9 x 5" loaf pan and line the bottom with parchment.
1. Sift together the dry ingredients. Gently toss elderberries into dry ingredients.
2. In a separate bowl, whisk together oil, eggs, vanilla, and cream. Add wet ingredients into flour mix and gently whisk together.
3. Finish mixing gently with a rubber spatula just until barely combined.
4. Pour batter into prepared pan and bake until golden and set, about 40-50 minutes. Allow to cool and remove from pan.
First make the plum puree and elderberry juice:
1. Place plums in a stainless steel pot with a little bit of water to prevent scorching. Cook until the fruit breaks down, about 15 minutes. Remove from pan, puree in a food processor, and strain through a sieve to remove skins. You need 1 cup of puree for this recipe.
2. For the berries, put about one pound (3 hefty cups) into a pot with a little water and cook until broken down, about 10 minutes. Pour berries into a jelly bag and allow to sit for an hour or so, until all juice has been removed. You need 1/4 cup of juice for this recipe.
Then, make the curd:
3. In a stainless steel bowl, whisk together egg yolks, plum puree, elder juice, sugar, and lemon juice. Fill a pot with about an inch of water and bring it to simmering. Place bowl of curd mix on top.
2. Cook, whisking frequently, until the curd thickens, about 20 minutes. It should hold its shape on a spoon.
3. Remove from stove, whisk in butter, and then strain. Chill immediately and allow to completely cool.
1. Place sugar in a sauce pan. Add enough water to just barely cover and stir gently to combine. Place over medium heat on the stove.
2. Meanwhile, start whipping your egg whites in a stand mixer with the whisk attachment on medium-high speed. You want your whites to reach medium peaks about the same time your sugar is ready. If they reach this stage before the sugar is ready, turn to low but keep mixing.
3. Cook sugar until it reaches about 240 F. It should be at the "thread" stage. Remove from heat and allow to cool for a few seconds until the bubbling subsides.
4. With egg whites mixing on medium, pour your sugar syrup in very slowly. Once all the syrup is in, allow the meringue to continue mixing for a while until it slightly cools.
5. Add butter, about one tablespoon at a time. Your mixture may look like it's separating for a while, but it will all come together in the end. Once all the butter is in and the mixture is smooth and creamy, add the vanilla.
1. First slice off any dry pieces near the edges and slice the top to make it flat. Slice cake horizontally in half.
2. Spread liberally with the curd.
3. Place second half on top and crumb coat with the buttercream. Chill for at least 20 minutes.
4. Finish frosting with remaining buttercream.
For an ombre effect, set aside two small bowls of frosting and color to varying hues with the plum curd. Alternatively, you could color the whole batch with desired amount of curd.
Best served at room temperature.