Place first four (dry) ingredients in a medium-sized saucepan, along with 1/2 cup of the milk and whisk to combine into a thick paste.
Add the remaining milk, bring the mixture to a boil, and whisk to combine well. When sauce boils, reduce heat to low and simmer very gently for 5 minutes, whisking often. Remove from heat, add vanilla, and cool.
Store in refrigerator for up to 2 weeks, covered.
BONUS TIP: May be used hot or cold; on ice cream, cake, fruit, or combined with milk to make a quick cup of hot chocolate (blend milk and amount of sauce to your own taste).add orange liqueur and a bit of orange zest for a chocolate orange sauceadd a couple of tablespoons of bourbon and some cherries for a cherry bounce chocolate sauceadd some mint extract and chopped fresh mint leaves for a chocolate mint sauceadd some crushed peppermint candy for a chocolate peppermint sauce