How to do it…

1. Combine dry ingredients in saucepan. If you don't like lumps, make sure to break up any clumps of dry ingredients.
2. Add water, whisking together as you do to incorporate wet and dry. I have Nigella's mini-whisk, which is awesome for this. Or if you have a sauce whisk, that's awesome too!
3. Cook on medium heat until thick, stirring constantly (you may need to add bits more cornstarch to thicken, be sure to whisk them in carefully for a smooth fudge) . When I'm making this, I tend to want to eat it, so I get impatient and add more cornstarch. You do you.
4. Now, you can toss in the butter when this hot mess of deliciousness starts thickening or you can do it at the end. I do it at the end when it starts to get thick. Whichever, toss that stick o' butter in (or slice it in for faster melting) and keep stirring to incorporate as it melts.
5. Once thick and all butter is incorporated, add vanilla and whisk in.
6. Serve hot or cold over individual servings of ice cream. Then add whipped cream, chopped nuts and a cherry. I mean, you don't have to do this last, but what's the point of a hot fudge sundae unless you go whole hog? My advice: make it an occasion and go whole hog!