First prepare the pears by thinly paring off the outer skin and placing them in a saucepan with the ginger ale, sugar, and cinnamon sticks.
Bring up to simmering point and gently simmer for about an hour until the pears are tender.
Remove from the liquid and allow to cool.
Halve the pears and remove the core and pips with a sharp knife.
Slice the ginger cakes and place evenly at the bottom of the trifle dish.
Splash the rum and stem ginger syrup evenly over the cake and cover with a layer of poached pears. Cover with clingfilm and place in the fridge while you make the custard.
Place the milk and single cream in the medium saucepan and bring to just before the boil.
Immediately remove from the heat and allow to cool a little.
Place the egg yolks, sugar, cornflour and vanilla extract in the mixing bowl and beat with the electric hand whisk until thick and pale in colour.
Pour the milk and cream into the egg mixture, whisking as you pour.
Clean the saucepan thoroughly, pour in the custard and place on a moderate heat.
Stir with a wooden spoon until the custard has thickened. It will thicken very suddenly so keep your eye out.
Remove from the heat as soon as it thickens and whisk with the electric hand whisk for a minute or two.
Cover with clingfilm and leave to cool for about 5 minutes.
Take the trifle out of the fridge and pour over the custard.
Cover and return to the fridge.
Once the custard is cold and set whip the double cream to soft peaks and spread over the custard.
Decorate with slices of preserved ginger and serve (or return to the fridge until needed).
