Over low heat melt butter thoroughly in a saucepan.
Add sugar, milk, and cocoa. (You can also add a little vanilla–1 tsp. or so)
Whisking briskly, bring to a boil–you can bring the temperature up to low/medium heat. Boil for 5 minutes, whisking constantly–it will make sure all the little lumps are removed!
Remove from stove, then serve warm or cold. It will thicken even more as it cools.
Keep refrigerated…will last 1-2 weeks. Cool in jars before placing in the refrigerator. One batch makes 2 cups, and fits into a pint-sized mason jar.
To warm, remove the metal lid and ring, and heat in the microwave for 30 seconds. Take it out and stir, then heat in additional 10 second increments until warmed through.
Notes
TIP: This recipe is great for doubling, tripling…I've even made up to 8 batches at one time! One batch makes 2 half pint jars.