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Heat oven to 200c/180c fan/gas 6. Grease the tin with a little oil and line the base with baking parchment. Put the eggs, caster sugar and a pinch of salt in a bowl. Whisk with an electric hand whisk until thick and doubled in volume – this will take about 5 mins. When ready, you should be able to lift out the beaters and the ribbons of batter that fall back to the bowl should remain visible on the surface for a few seconds.
Sift in the flour and baking powder, then use a large metal spoon to fold in until just combined. Working quickly, tip half the mixture into another bowl. Add the lemon zest and yellow food colouring to one bowl, and the vanilla and orange colouring to the other. Mix again until just combined – try not to overmix as this will knock out the air, giving you a flat sponge. Transfer each mixture to a piping bag.
Snip the end off of one piping bag, about 2cm from the tip. Quickly pipe lines along the tray. Repeat with the other bag, then bake for 10 mins.
While the sponge cooks, spread a tea towel on your work surface with a sheet of baking parchment on top. When the sponge is cooked, flip it onto the parchment, then flip the whole thing over again, so that the brighter side of the sponge is now on the bottom. Using a sharp knife, trim the edges to neaten, then, with the shorter end facing you, use a sharp knife to score along one edge, about 2cm from the end – this will help to roll it up. Tuck the scored edge into the sponge, then continue rolling away from you, rolling the parchment and tea towel with it as you go. leave to cool completely like this for 20 mins.
To make the filling, mix the mascarpone with the icing sugar and lemon juice. Once cooled, unroll the sponge and remove the baking parchment. Spread the mascarpone over the inside of the sponge, top with blobs of lemon curd, then re-roll, dust with a little icing sugar and serve. Will keep in the fridge for 2 days.