Blueberry and Mascarpone Swiss Roll

STEP 1: MAKE THE TARTAN INLAY PASTE

Beat the softened butter and icing sugar until smooth and pale. Beat in the egg white until combined (it may look split — that's fine), then fold in the flour to a smooth, pipeable paste. Divide between two or three small bowls and tint: one forest green for the dominant ground stripe, one navy or royal blue, and — if you like — a scrap deepened with black for the finest over-check lines. Spoon each into a small piping bag fitted with a fine (2–3 mm) plain nozzle.

STEP 2: PIPE THE TARTAN AND CHILL

Line the Swiss roll tin with parchment, pressing into the corners. On the parchment, pipe the plaid: a set of evenly spaced green lines down the length of the tin, then a second set crossing it widthways; add paired blue lines running alongside the green in both directions and, if using, the thinnest black over-checks to complete the sett. Pipe confidently — a little irregularity reads as woven cloth. Freeze the tin for 10–15 minutes so the paste firms and holds crisp lines. Meanwhile, preheat the oven to 425°F (220°C).

STEP 3: MAKE THE GREEN SPONGE

In a heatproof bowl set over barely simmering water, whisk the eggs and caster sugar until just warm and the sugar has dissolved. Off the heat, whisk on high with an electric mixer for 5–7 minutes to the ribbon stage — thick, pale, tripled in volume, holding a trail for several seconds. Whisk in the vanilla and enough forest-green gel to reach a vivid green (it dulls slightly on baking, so go a shade bolder). Sift the flour and salt over in two additions, folding gently with a large metal spoon to keep the air; fold in the cooled melted butter last, if using.

STEP 4: BAKE THE INLAID SPONGE

Take the chilled, patterned tin from the freezer and gently pour the green batter over the frozen tartan paste, coaxing it into the corners and levelling with an offset spatula without dragging the lines. Bake for 8–10 minutes, until springy and just set — do not overbake, or it will crack when rolled. While it bakes, lay a fresh sheet of parchment on the counter and dust lightly with caster sugar.

STEP 5: ROLL WHILE WARM

Invert the hot sponge onto the sugared parchment so the tartan face is downward, and peel away the baking parchment to reveal the pattern. While still warm, roll the sponge loosely from a short end with the fresh parchment coiled inside, and leave to cool completely in its rolled shape. This single step — rolling warm — trains the sponge to re-roll later without cracking.

STEP 6: MAKE THE BLUEBERRY COMPOTE

Combine the blueberries, sugar, lemon juice and zest in a small saucepan over medium heat and simmer for 6–8 minutes until the berries burst. Stir in the cornflour slurry and cook 1–2 minutes more until thick and glossy; crush some berries while leaving others whole for texture. Cool completely, then chill — the compote must be cold and set, not runny, or it will weep into the sponge.

STEP 7: WHIP THE MASCARPONE CREAM

In a chilled bowl, whisk the cold mascarpone, double cream, icing sugar and vanilla on medium speed until it holds firm, pipeable peaks. Stop the moment it thickens — mascarpone splits easily if overwhipped. Keep cold.

STEP 8: FILL AND RE-ROLL

Gently unroll the cooled sponge (it will keep a slight curl). Spread the mascarpone cream evenly over the surface, leaving a 1/2 in (1 cm) clear border at the far edge. Spoon the chilled blueberry compote over the cream in a lengthways band, or dot and lightly streak it through for a marbled swirl. Using the parchment to help, re-roll firmly but without squeezing out the filling, and finish seam-side down.

STEP 9: CHILL, TRIM AND SERVE

Wrap the roll in parchment and chill for at least 1 hour to set the filling and firm the shape. Trim a thin slice from each end with a hot, sharp serrated knife (dipped in hot water and wiped between cuts) to reveal the clean tartan spiral and the blueberry-and-cream swirl. Dust lightly with icing sugar if you like, and serve cold.