Method

Make a bain-marie with a heatproof bowl (glass in best) & a large saucepan, try to find a bowl which goes down into the pan by about a 3rd the way down, or use a double boiler if you have one.

Pre-heat fan oven to 160'c to sterilise jars, wash the jars first in warm soppy water, rinse under the tap then place in a baking tray & put in the over for 10 mins to heat & dry, the lids & any rubber parts place in a saucepan with boiling water for a 2 to 3 mins before use.

In the bowl add egg & yolk, caster sugar, both zests, cornflour & salt & whisk with a balloon whisk or a electric hand mixer until light & fluffy.

Next whisk in slowly the orange & lemon juices in the the mixture, place the bowl on the pan of simmering water, don't allow the water to touch the bottom of the bowl, whisk the mixture for about 10 mins or until the foam goes & it thickens & coats the back of a spoon, then take bowl off the pan & keep whisking then add the butter a few cubes at a time until all in.

Sieve out the zest if you like it smooth & pour into hot jars from the oven on a baking tray on the worktop & fill to 1 cm from the top of the jar with a jam funnel, then place on the sterile lid, & set aside to cool.