Preheat the oven to 200°C Fan. Line two baking trays (one small and one large, see equipment list above) with baking parchment.
Whisk the eggs, vanilla paste and sugar using an elecric whisk, until pale, fluffy and leaves a ribbon trail when lifted out (see photos above). It may take 7-10 minutes of whisking to get to that stage.
Add the sifted flour and fold it in carefully, trying not to knock out too much air.
Divide the batter between the two lined trays and level out using a spatula. Make sure the batter gets into all four corners.
Bake for 10-12 minutes. The sponges should be golden brown and slightly risen.
Whilst the sponges are cooking, get two large sheets of baking parchment. Place them on the worktop and sprinkle them with caster sugar, ready for the sponges to be turned out on.
Spoon lemon or lime curd into a bowl and add some green food colouring. Mix thoroughly. You don't have to use any colouring, but if you want your spirals to stand out on a finished Charlotte Royal, you need your curd to be a significantly different colour to your sponges.
When the sponges are baked, take them out of the oven and carefully turn them out onto prepared baking parchment sheets (so they are upside down). Peel off the parchment off the top of the sponges slowly and carefully.
Trim the short end of the sponges with a sharp knife to get a sharp edge.
Score the sponges on the short end 2 cm from the edge, but don't cut all the way through. This will help to get the roll started.
When the sponges have cooled slightly (they don't need to be cooled completely), spread the lime or lemon curd in a thin layer all over, the sponges.
Starting from the scored end, roll the sponges tightly, making sure not to squeeze out too much of the curd. You should now have two swiss rolls: one small and one large. Set them aside to cool completely.