MAKE THE THICK LEMON CURD (MAKE AHEAD)
1. In a heatproof bowl set over a saucepan of barely simmering water, whisk together the egg yolks, whole egg, sugar, lemon juice, lemon zest, and cornstarch until combined.
2. Stir continuously with a wooden spoon or silicone spatula for 8–12 minutes until the mixture is very thick — it should hold a visible trail when you lift the spoon and coat the back of it heavily. The cornstarch stabilizes the curd and ensures it sets firmly rather than running out when the roll is sliced.
3. Remove from the heat and add the cold butter cubes one at a time, stirring until each is melted and fully incorporated.
4. Strain through a fine-mesh sieve into a clean bowl. Press cling film directly onto the surface to prevent a skin forming. Refrigerate for at least 2 hours, ideally overnight. The curd should be spreadably thick when cold, not pourable.

MAKE THE SPONGE BATTER
5. Preheat the oven to 400°F (200°C). Line a Swiss roll tin (approximately 13×9 inches / 33×23 cm) with parchment paper.
6. Using an electric whisk, beat the eggs and caster sugar together for 8–10 minutes until extremely thick, pale, and mousse-like. When you lift the whisk, the trail should sit on the surface for at least 5 seconds before dissolving (the ribbon stage).
7. Beat in the lemon zest. Sift the flour over the surface in three additions, folding each in with a large metal spoon or spatula using gentle figure-of-eight motions. Do not stir or beat — preserve the air that gives the sponge its structure.

BAKE — WATCH THE CLOCK
8. Gently pour the sponge batter into the prepared tin, spreading evenly with an offset spatula.
9. Bake for 8–10 minutes only — until the sponge is golden, springs back when gently pressed, and is just starting to pull away from the edges. Set a timer and check at 8 minutes. Better slightly underdone than even 1 minute over.

ROLL IMMEDIATELY
10. While the sponge bakes, lay a clean tea towel on your work surface and sprinkle it generously with caster sugar.
11. The moment the sponge comes out of the oven, invert the tin onto the sugared towel. Peel off the parchment paper carefully.
12. Starting from a short edge, roll the warm sponge up tightly with the towel inside. This trains the sponge to hold its curl. Allow to cool completely in this rolled position, about 30 minutes.

MAKE THE MASCARPONE LEMON CREAM
13. In a cold bowl, whip the mascarpone, cream, icing sugar, lemon zest, and vanilla together until the mixture holds soft, pillowy peaks. Do not over-whip — mascarpone can turn grainy if overworked. Refrigerate until ready to use.

FILL AND ROLL
14. Gently unroll the cooled sponge. Spread a generous, even layer of thick lemon curd over the entire surface, leaving a 1 cm border at the far edge.
15. Spread the mascarpone lemon cream over the curd in a slightly thinner layer.
16. Re-roll the sponge without the towel, rolling firmly but not so tight that the filling squeezes out. The first turn should be the tightest. Place seam-side down on a serving plate.
17. Refrigerate for at least 30 minutes to set before slicing.

SERVE
18. Using a sharp serrated knife, trim both ends at an angle to reveal the spiral of golden sponge, bright yellow curd, and white cream. Dust lightly with icing sugar if desired. Slice with a gentle sawing motion.