This is very light, fresh and lemony, with a lovely squidgy centre.
If you have only one baking tin of the right size, you can make and bake one base at a time, by mixing half the base ingredients at a time. Don't make it as one big batch and bake them one after the other, as the raising agent will be lost as the mixture stands.
This recipe is from Delia's Complete How to Cook. Makes 2 roulades (serves 12-16)
Begin by making the lemon curd ahead of time, as this needs to be well chilled.
Sit a heatproof bowl over a pan of barely simmering water, but don't let the bowl touch the water. Then place the eggs, sugar, lemon zest and juice in the bowl and whisk together. Now add the butter piece by piece and cook for about 15 minutes, giving it a good whisk from time to time, until the curd has become smooth and thickened. While you're keeping an eye on it, it's a good time to line the baking tin. When the curd is ready, remove the bowl and allow it to cool, then cover it with clingfilm pressed over the surface and chill it in the fridge. When you're ready to make the sponge base, place all the ingredients except the icing sugar in a large mixing bowl and whisk, preferably with an electric hand whisk, for about 1 minute, then add 2 tablespoons of hot water and whisk briefly again.
Next divide the mixture between the prepared baking tins, smooth it out and bake in the oven for 8-10 minutes. You may need to swap the trays over, but don't open the oven door until 8 minutes have elapsed, or the sponges may sink. When the sponges are ready they should feel springy in the centre when lightly touched with your little finger. Then remove the sponges from the oven and let them cool in the tin for 5 minutes.
Meanwhile, lay a sheet of silicone paper (parchment), 18 inches (45 cm) long for each sponge, on a flat surface, then dust them all over with a little icing sugar. Then, taking each sponge in turn, carefully turn them out on to the parchment and gently strip away the base lining paper. Cover them with a damp tea towel and leave for another 5 minutes (but no longer, as it should still be warm to make the rolling easy).Don't panic, this really is quite easy: just lift the paper along the shorter edge and fold it over, then gently roll the sponge up with the paper tucked inside as a lining. If cracks appear, don't worry – they can look very attractive. Now leave the roulades to get cold.
When you're ready to fill them, carefully unroll them and spread each with a third of the lemon curd all over. Now whisk the mascarpone, fromage frais and sugar together and spread a third of that over each roulade. Then roll each roulade up carefully, but much more loosely. For the finishing touches, remove the zest of the 2 lemons with a zester so that you get nice curly strands, then place in a saucer and mix with about 1 tablespoon of the lemon juice. Next, in a bowl, mix the icing sugar with 2 tablespoons of the lemon juice to make a thin glaze. Drizzle this with a spoon, using sweeping movements back and forth, across the top of each cake.
Then drain the lemon zest and scatter half over the icing on each cake. Combine the leftover mascarpone mixture and lemon curd and serve the roulade cut into slices with the extra lemon mascarpone cream handed round separately.The roulades can be made and decorated in advance, covered loosely with foil and kept in the refrigerator. Remove them ½-1 hour before serving.