I always asked for this on my Dairy Queen sundae so I tried making it at home. It's super easy and I think my version tastes even better. My secret ingredient is sweetened condensed milk. It makes the sauce super creamy and slightly sweet.
Whisk all the ingredients in a small bowl until smooth. If you find the sauce is too thick, you can add a little bit extra water. The sauce will last for about one week in the fridge. You can also freeze it and defrost it for 30 minutes before using it.