Put powdered milk, vanilla bean (split in two and seeds scraped out in the pot) and sweetener in a teapot.
Pour 2 tbsp of the boiling water in the teapot and mix well with a spoon, you should get a smooth paste (it is important that you do this or the powdered milk might sort of "curdle", don't ask me why but it does it!).
Add the teabag(s) to the teapot and pour remaining boiling water, mixing well; let steep 5-7 minutes (or longer if you like a stronger tea flavour).
Strain to get rid of the vanilla pieces (you don't need to do it if you use vanilla essence, of course).
If you have a milk "frother" you can use it to have a lightly foamy version (I suggest you do it directly in the serving mug/cup, or the light froth won't last).
A stronger-adult version can be made by substituting vanilla bean/essence with a vanilla flavoured liqueur.
Or, you can just add it.
Sip preferably on a comfy couch, cuddled up under a blanket with a good book, a pet, and a nice background music-- enjoy!