Hey folks, 

Just wondering if anyone has a reliable method and recipe for a mirror glaze and how long do they keep for? 

I'm thinking of doing it for our Christmas cake 

Hi,

This is my go-to, tried and tested chocolate mirror glaze recipe (scroll down towards the middle of the post):
https://thatbakergirlblog.wordpress.com/2016/06/09/tangerine-cake-with-chocolate-cream/

I usually make it a couple of days ahead and keep it in the fridge, then reheat it in the microwave until it reaches pourable consistency. It can keep for at least 3-4 days refrigerated. I prefer this one to most mirror glazes made with sweetened condensed milk because it has a bitter chocolate flavor that cuts the richness of the buttercream or other sweet fillings.

If you want to color it (or if you're looking for a simpler version), then I recommend Elise's recipe:

http://mycupcakeaddiction.com/en/recipes/mirror-cakes-made-simple

Hope this helps :)