Place 4 glasses or glass serving dishes in the fridge to chill well. Preheat the oven to 100C/80C Fan/Gas ¼.
For the dried rhubarb crisp, using a sharp vegetable peeler, carefully shave the rhubarb lengthways until you have at least 6 pieces, then put these in a small bowl. Chop the remaining rhubarb into 1cm/½in pieces and set aside.
In a small saucepan over a high heat, bring 100ml/3½fl oz water, the sugar and grenadine syrup to a gentle simmer. Add the shaved rhubarb slices, then turn off the heat and leave to cool for 10 minutes.
Drain the slices of rhubarb, reserving the syrup, and place on a baking tray lined with a silicone mat. Place in the oven for 25 minutes to dry out, then store in an airtight container until needed. Increase the oven temperature to 140C/120C Fan/Gas 1.
To macerate the rhubarb, in a large bowl mix the rhubarb (including the reserved chopped rhubarb) and sugar with 100ml/3½fl oz of the reserved rhubarb syrup and leave to macerate for 30 minutes. Transfer to a shallow oven tray and cover tightly with cling film to create a seal. Place in the oven for 25 minutes. Once cooked, transfer to the fridge to cool in its own syrupy juice before straining this off and reserving it. Finely chop half of the cooked rhubarb (about 180g/6oz), leaving the rest as it is, and set aside in 2 separate bowls.
Measure 200ml/7fl oz of the reserved syrupy juice to make the jelly.
For the jelly, in a small saucepan over a low heat, infuse the ginger and softened gelatine in the reserved juice for 5 minutes. Stir to dissolve the gelatine. Strain, reserving 80ml/2¾oz for building the trifle. Pour the remainder into a small bowl, cover and chill in the fridge.
Start to build your trifle before you make the custard. Place 40g/1½oz of the finely chopped cooked rhubarb in the bottom of each chilled glass. Top with 50g/1¾oz of the larger pieces of cooked rhubarb. Add 4 teaspoons of the reserved jelly liquid into each glass and place in the bottom of your fridge for 15 minutes, until the jelly has just set.
For the custard, in a medium saucepan over a medium heat, bring the cream, milk, sugar and vanilla purée to the boil. In a large bowl, whisk the egg yolks.
Once the cream mixture comes to the boil, pour it over the egg yolks and whisk until evenly distributed. Pour this mixture back into the pan and bring it to 75C on a sugar thermometer, stirring all the time. Take off the heat and place the pan in a bowl of ice-cold water. Using a hand-held blender, blitz in the softened gelatine and continue to blend until the custard is cool. (By whisking as it cools you will add a third more volume thanks to the air that is incorporated.)
It is important that the custard sets in the glass you are going to serve it in or you will lose all the air bubbles and ultimately the lightness of the dessert. So, take each of the desserts from the fridge, top with custard, cover and return to the fridge to set for at least 3 hours, or overnight if you have the time.
Meanwhile, prepare the honeycomb. Line a 20x25x3cm/16x18x1in baking tray with a silicone mat. Put 40m/1½fl oz water, and the honey, glucose and sugar in a deep, medium-sized saucepan and leave for a minute. Place the pan on a high heat and bring to the boil. Reduce to a medium heat and cook for 5 minutes, until you have a light golden-brown caramel.
Take off the heat and stir in the bicarbonate of soda. (CAUTION: There will be a huge amount of frothing and bubbling and it will increase in volume by about 5 times. This is caused by the bicarbonate of soda reacting with the heat of the sugar, which produces the bubbles that will eventually create a crunchy honeycomb. Beware – it is a very pretty spectacle but children shouldn’t be allowed close.) Once the frothing has stopped, stir well to ensure that the bicarbonate of soda is well mixed, then carefully pour it into the lined tray. Allow to cool before turning out and breaking into pieces. Store in an airtight container until needed.
To serve, top each dessert with some broken honeycomb, small spoonfuls of the reserved jelly, one of the rhubarb crisps and the angelica, if using, which will add a little more magic to this dish.