Put the condensed milk, milk, soft dark brown sugar, and butter into a pan.
Turn the heat to low and allow everything to melt together whilst stirring constantly - DO NOT let it stick to the pan. It’s really easily done, so just keep stirring.
Once everything has melted, turn the heat up slightly and keep stirring until the mixture is bubbling.
Put the sugar thermometer into the pan - the fudge mixture needs to be 116°C which will take between 10-15 minutes. REMEMBER TO KEEP STIRRING!
Once the fudge reaches 116°C, turn off the heat and transfer it to the bowl on your stand mixer - be careful as the fudge will be extremely hot.
Add the Christmas pudding.
Set your stand mixer to the highest setting and whisk for about 10 minutes.
Transfer into a square cake tin lined with greaseproof paper.
Leave to set overnight at room temperature and then cut into bite-sized chunks.