Lemon and Blueberry Vertical Layer Cake

STEP 1: MAKE THE BLUEBERRY REDUCTION
Combine the blueberries, granulated sugar and lemon juice in a small saucepan over medium heat. Simmer, mashing the berries, for 10–15 minutes until thick, glossy and reduced by more than half — it should mound on a spoon rather than run. For a smooth result, push it through a sieve, then cool completely. This real reduction (not essence) is what gives the buttercream genuine blueberry depth rather than a pale tint.

STEP 2: BAKE A THIN, EVEN SHEET SPONGE
Preheat the oven to 180°C (350°F). Line a 33 × 46cm (13 × 18-inch) rimmed baking sheet with parchment. Whisk the eggs and caster sugar over a pan of simmering water until warm, then whip off the heat for 5–7 minutes to the ribbon stage. Sift over the flour and salt in two additions and fold gently; fold in the cooled melted butter, lemon zest and vanilla last. Spread to an even, thin layer and bake just 12–15 minutes until springy. A slightly underbaked-feeling sheet stays flexible and resists cracking along the coil.

STEP 3: ROLL THE SPONGE WARM TO SET ITS MEMORY
Turn the hot sponge out onto a sugar-dusted sheet of parchment, peel off the lining, and while it is still warm roll it up loosely from the short edge, parchment and all. Let it cool fully rolled. Rolling warm trains the sponge into a curve, so that when you unroll, fill and re-coil it, it bends without snapping — the single most effective guard against the vertical crack.

STEP 4: MAKE THE BLUEBERRY BUTTERCREAM
Beat the softened butter until pale and creamy, 3–4 minutes. Add the sifted icing sugar in additions, then beat in the cooled blueberry reduction (reserving 1 tablespoon) and a pinch of salt until smooth and spreadable. If it loosens, chill for 10 minutes and re-beat.

STEP 5: CUT, FILL AND BUILD THE VERTICAL SPIRAL
Gently unroll the cooled sponge. Trim into two or three long strips of equal width (roughly 5–6cm / 2–2.5 inches tall). Spread a thin, even layer of blueberry buttercream over each strip — thin layers roll more reliably than thick ones. Roll the first strip into a tight pinwheel, stand it cut-side up on the serving plate, then wrap the next strip around it, joining the ends, and continue until you have one tall spiral of the size you want. Chill for 20 minutes to firm before icing.

STEP 6: MAKE THE SHARP LEMON CREAM-CHEESE ICING
Beat the softened butter smooth, then beat in the icing sugar. Add the cold, well-drained cream cheese and beat only until just combined — do not overwork or it will slacken. Fold through the lemon zest and enough lemon juice to keep it genuinely sharp against the sweet blueberry. Drained, cold cream cheese and minimal mixing give an icing firm enough to hold the tall sides cleanly rather than sliding.

STEP 7: COAT, FINISH AND SLICE
Spread a thin crumb coat of the lemon icing over the chilled spiral and chill for 15 minutes, then apply a final smooth coat up the vertical sides and across the top. Swirl the reserved tablespoon of blueberry reduction through the top for a marbled finish, and add a few fresh blueberries and a curl of lemon zest to signal the flavours within. Slice with a hot, dry knife to reveal the upright stripes of sponge and purple buttercream.