Rhubarb and Custard Meringue Roulade

MAKE THE CRÈME PÂTISSIÈRE (the day before, or 2 hours ahead):
1. Combine the milk and split vanilla pod (with seeds scraped in) in a saucepan and bring just to a simmer; turn off the heat and infuse for 10 minutes.
2. In a bowl, whisk the egg yolks, sugar and cornflour to a smooth paste.
3. Pour the warm milk through a sieve over the yolk mixture in a slow stream, whisking constantly.
4. Return everything to the pan and cook over medium heat, whisking without pause, until thickened and one or two large bubbles plop on the surface (about 90 seconds at the boil).
5. Off the heat, whisk in the cold butter.
6. Press cling film directly onto the surface and chill until completely cold — at least 2 hours.

MAKE THE RHUBARB COMPOTE:
7. Cut the rhubarb into 2 cm pieces.
8. Combine in a saucepan with the sugar, lemon zest, lemon juice and vanilla seeds.
9. Cook over medium-low heat, stirring once or twice, for 8–10 minutes until the rhubarb has softened and released its juice but the pieces still hold their shape.
10. Tip into a sieve set over a bowl and let drain for 10 minutes.
11. Reserve the strained syrup separately - you will brush it on the meringue.
12. Cool the compote completely before assembly.

PREPARE TO BAKE THE MERINGUE:
13. Heat the oven to 180°C/350°F (160°C/325°F fan).
14. Line a 23 × 33 cm (9 × 13 in) Swiss roll tin with parchment, leaving 2 cm overhang on every side.
15. Have the cornflour, vinegar, vanilla and flaked almonds weighed and ready before you start whisking - meringue does not wait.

WHISK AND BAKE THE MERINGUE:
16. Whisk the egg whites on medium speed until they hold soft peaks.
17. With the mixer running, add the caster sugar one tablespoon at a time, waiting 10–15 seconds between additions.
18. Once all the sugar is in, increase to high speed and whisk for 5–7 minutes until the meringue is glossy, holds firm peaks, and feels completely smooth between your fingertips with no sugar grit.
19. Sift the cornflour over the meringue, drizzle in the vinegar and vanilla, and fold them through with a spatula in 6–8 decisive strokes.
20. Scrape the meringue onto the parchment-lined tin and spread to a roughly even rectangle, smoothing the top with an offset spatula.
21. Sprinkle the flaked almonds evenly over the surface.
22. Bake at 180°C/350°F for 25 minutes, then drop the oven to 160°C/325°F and bake for a further 5–7 minutes. The meringue should be set firm to the touch with a thin crisp crust on top and barely-coloured almond flakes; the interior will still be tender and marshmallowy.

COOL AND PREPARE FOR ROLLING:
23. Cool in the tin for 10 minutes.
24. Lay a fresh sheet of parchment on the bench, dust generously with icing sugar, and invert the meringue onto it (almond side now down against the sugared parchment).
25. Carefully peel away the original baking parchment from what is now the top.
26. Brush the exposed surface lightly with the reserved rhubarb syrup - not enough to soak, just enough to colour and to keep the surface supple.

FINISH THE CRÈME DIPLOMAT:
27. Whip the cold double cream to soft peaks.
28. Whisk the chilled crème pâtissière smooth (it will look broken until you do), then fold the whipped cream through in two additions. The finished diplomat should be thick enough to hold a piped ribbon but soft enough to spread without tearing the meringue.

FILL:
29. Spread the crème diplomat in an even layer over the meringue, leaving a 1.5 cm border on all four sides.
30. Spoon the drained rhubarb compote in a continuous ribbon down the centre, parallel to the short edge closest to you (this is the line you are about to roll towards). Do not spread the rhubarb across the whole surface - a concentrated central band gives the spiral its visible rhubarb-and-custard alternation.

ROLL (WIDTHWAYS, WITH PARCHMENT TENSION):
31. Orient the meringue so the long edges are at your left and right and the short edges are at the top and bottom.
32. Score a shallow line 2 cm in from the short edge nearest you - just deep enough to break the crust, not into the meringue.
33. Lift the parchment under the near short edge and use it to fold the first 2 cm over onto itself, creating a tight starting coil.
34. Now roll forwards across the short edge in a single confident motion, keeping firm tension on the parchment as you go to compress the spiral. The finished roulade should show 3–4 visible coils when sliced.

PIPE AND BAKE THE MERINGUE KISSES:
35. Whisk the single egg white to soft peaks, then add the sugar a tablespoon at a time and whisk to glossy stiff peaks.
36. Tint with one or two drops of pink colouring if using.
37. Transfer to a piping bag fitted with a small plain round nozzle and pipe small kisses (about 1.5 cm across) onto a parchment-lined tray.
38. Bake at 100°C/210°F for 60–75 minutes until they lift cleanly off the parchment, then cool. (You can bake these the day before.)

DECORATE, CHILL, SERVE:
39. Transfer the rolled roulade to a serving board.
40. Stick the meringue kisses to the top of the roulade in a line down its length, anchoring each one with a tiny dot of the leftover diplomat or a smear of jam.
41. Refrigerate for at least 1 hour before slicing — this firms the diplomat and gives you the clean spiral cross-section.
42. Slice with a hot, dry knife (dip in hot water and wipe between cuts). Serve cold, on its own or with a small extra spoon of the strained rhubarb syrup as a glossy garnish.