Add the powdered sugar in a little at a time (I usually do this in 4 steps). Mix on low until the sugar begins to combine with the butter, then turn the mixer up to high. After the second addition of powdered sugar, add in the reduced pomegranate juice and the salt. Beat until fully combined. Add in the rest of the powdered sugar a little at a time, starting on low and increasing the speed to high as the sugar begins to incorporate. If your buttercream is a bit stiff, add in the milk 1 Tbsp at a time until you have a smooth and pipe-able consistency.