When the sponge-jelly layer is set, make the crema (custard) layer. Fill your sink with a few inches of cold water. Heat the milk in a large saucepan over a medium-low heat until warm. Meanwhile, whisk the egg yolks with 60g of caster sugar and the cornflour. Finely grate in the remaining lemon zest. Pour a ladleful of the warmed milk onto the eggs and quickly whisk it in. Repeat with another ladle of milk until the mixture loosens. Pour the egg mixture into the pan of warmed milk and whisk constantly until thickened. You’ll need to be vigilant that it doesn’t overcook and scramble. 

Pour the cream through a sieve into a large mixing bowl and stand this in the cold water in the sink (making sure no water gets into the bowl!) until it has cooled completely. Whisk it as it is cooling down so that a skin doesn’t form. When it is completely cool, carefully pour it over the jelly layer. Cover the dish and put it back in the fridge to set.

To make the praline, preheat your oven to 180ºC/gas mark 4. Scatter the walnuts on a small baking tray or dish and place in the oven for 8 –10 minutes, or until lightly browned. Line a baking tray with greaseproof paper. Pour the remaining 100g of sugar into a large non-stick frying pan and place on a medium heat. Melt the sugar, swirling the pan as it melts, until it is completely molten and a deep caramel colour. Working quickly, scatter in the walnuts, crush in a pinch of salt and pour onto the greaseproof paper. Leave it to set.

When you are ready to serve the trifle, whisk the double cream until you have soft peaks, then spoon it over the set crema. Smash the walnut caramel in a pestle and mortar till you have a mixture of fine and coarse pieces and scatter over the cream (you’ll probably have extra, just save it to sprinkle over ice cream for another day). Serve straight away.