I made my first vertical layer cake years ago. While it was visually striking, the cake resembled something closer to cardboard than a confection, and the filling was overly sweet and artificial. This cake, however, not only tastes of floral, sweet, and tart notes, it is delightful from the inside out. Blackberry and elderflower is the perfect pair for any springtime celebration. The flavor is light enough to take you all the way through the peak of summer when blackberries are plump and at their sweetest. It’s a true showstopper even after it is sliced!
No, this is not an optical illusion. By flipping this jelly roll–style cake up on its side, each slice reveals vertical layers as opposed to horizontal ones. The naturally hued blackberry buttercream contrasts with the light cake to make the stunning stripes within. This blackberry cake is as delicious as it is dazzling!
Cook’s Note: If working with two cakes at the same time seems intimidating or you only have one large baking sheet, cut the cake recipe in half and bake them one at a time.
Vanilla Sponge Cake
Whipped Vanilla Buttercream
Preheat the oven to 350°F. Grease two 13 by 18-inch pans and line the bottoms with parchment paper (see notes).
In the bowl of a stand mixer fitted with the whisk attachment, beat the eggs and granulated sugar on medium-high speed until the mixture triples in volume, about 8 minutes. Stop the mixer and sift in the flour, cornstarch, baking powder, and salt. Using the mixer’s whisk, stir the mixture by hand until combined.
In a separate bowl, stir together about 1 cup of the egg-flour mixture and the melted butter. Add the vanilla, pour the butter mixture back into the bowl with the remaining egg-flour mixture, and gently fold to thoroughly combine.
Evenly divide the batter between the prepared pans and bake for 12-15 minutes, until the surface springs back when gently touched. Rotate the pans halfway through baking. Do not overbake, or the cakes may crack.
Dust the warm cakes with a generous amount of confectioners’ sugar. Place a clean tea towel on top of each cake. carefully flip the pans upside down to release the cakes onto the towels. Peel off the parchment paper and dust the bottoms of the cakes with confectioners’ sugar. While the cakes are still warm, roll them up in the towels, starting at the short ends. Set the cakes seam-side down and let cool completely in the towels, 30-45 minutes. once cooled, carefully unroll the cakes, and remove the towels. Like muscle memory, the cake will “remember” to retain its curved shape once cooled.
In a small saucepan, combine the sugar, elderflower liqueur, lemon juice, and 2 tablespoons water and bring to a boil. reduce the heat to low and simmer for about 5 minutes. remove from the heat and set aside to cool before use.
In a small saucepan, combine the blackberries and lemon juice and bring to a simmer over medium heat, 5-10 minutes. using a submersion blender, puree the cooked blackberries (be careful, as this can be messy). Alternatively, smash the blackberries with a potato masher or the back of a slotted spoon. Strain the mixture through a fine-mesh sieve set over a bowl. Press down on the puree with a rubber spatula to extract all the juice. discard the solids in the strainer. Set the strained puree aside and cool before use.
With a handheld mixer or using a stand mixer fitted with the paddle attachment, beat together the buttercream and 4-6 tablespoons of the blackberry puree, a few tablespoons at a time. The buttercream should be soft and spreadable but not runny.
To create the vertical layer cake, cut the cooled cakes in half lengthwise to create 4 long strips of cake. Generously brush the strips of cake with the elderflower syrup. Spread the blackberry buttercream on top of each strip of cake with a small offset spatula, using about ¾ cup of the buttercream per strip of cake. Gently roll up one strip to make a spiral. Turn the rolled strip on its side and begin wrapping the remaining cakes around the center spiral, setting the start of each strip at the end of the last one to create a large spiral of cake and filling. Place the cake on a cake board or serving plate. crumb coat the cake with the buttercream and chill in the refrigerator for 15 minutes.
Set aside about ½ cup of the remaining buttercream. Remove the cake from the refrigerator and smoothly frost with the blackberry buttercream. Tint half of the reserved buttercream with a bit of the blackberry puree or gel food colouring to create two shades of purple.
To create the buttercream flowers, fill a piping bag fitted with a medium closed-star tip (such as Wilton #2B) with the two shades of remaining buttercream. Use a small offset spatula to place the two shades on either side of the piping bag (it is okay if they aren’t perfectly divided—the swirl of color will give the piped flowers more dimension).
Holding the piping bag perpendicular to the surface of the cake, pipe clusters of flowers on the top and sides of the cake, or as desired. Use the same technique as piping a star. Place a sugar pearl in the center of each flower.
If eating the cake the same day as assembled, store it at room temperature until ready to serve. If assembled in advance, store in a cake box in the refrigerator overnight. Bring to room temperature for 30 minutes before serving.
Store leftovers loosely covered with plastic wrap in the refrigerator for up to 3 days.
In the bowl of a stand mixer fitted with the paddle attachment (or in a large bowl using a handheld mixer), beat the butter on medium speed until smooth and creamy.
With the mixer running on low, slowly add all but 1 cup of the confectioners’ sugar, the milk, and vanilla. Once incorporated, turn the mixer up to medium-high and mix for 3-5 minutes, until the buttercream is white, fluffy and smooth. Add the remaining 1 cup confectioners’ sugar as needed 1⁄3 cup at a time, until the desired consistency is reached; the buttercream should be soft and spreadable, but not runny.
Whipped vanilla buttercream can be made in advance and stored in a lidded container or wrapped tightly in plastic at room temperature up to overnight, in the refrigerator for 1-2 weeks, or in the freezer for up to 3 months. Bring the buttercream to room temperature and mix until smooth before using.