Make sure the butter and cream cheese are both at room temperature, then beat them both together until creamy and smooth.
Sift the powdered sugar into the bowl, then mix it in until there are no lumps.
Add the vanilla and a pinch of salt if desired. Mix until incorporated.
Check the consistency of your glaze. If you want a more runny glaze, add 1-2 tablespoons of milk until the desired consistency is reached. If you want a thicker glaze, or added too much milk, you can add a little more powdered sugar to thicken it up again.
Notes
If you are spreading or piping your glaze onto something like cinnamon rolls or a bundt cake, you will probably want it as is.If you are drizzling on something like loaf cake or muffins, you will likely want to add a little milk to loosen it up. Tip: If you are pouring onto a cake and want to control the drip more, I find it easiest to put the glaze into a piping bag so I can drip it exactly where I want it to go.