The second attempt at this recipe was much more successful. I altered the recipe by adding the zest of a lemon to the whole cake batter, as well as the vanilla extract, and a few drops of green colouring to the pistachio and lemon curd part of the batter. Other than that, I followed the same recipe as before. Once again, I made the lemon curd myself, to this recipe but using more lemon zest; I looked at commercial lemon curd but even the most expensive brands had added gelling agents and I wasn't sure how they would react to heat.
This time the lemon flavour was stronger and adding some food colouring showed up the ripple effect much better, as well as proving that the lemon curd portion of the batter had stayed separate. I'm a little dubious about the green colouring; it used to be said that people didn't like to eat food which had been artificially coloured blue or green, because those colours are associated with mould, but I'm not sure that's true these days.