In an elecrtic mixer, on low speed, cream the butter for a minute.
Add 3.5 cups icing sugar to the butter and mix on low until the sugar is incorporated.
Add salt, vanilla, and 2 tbs milk or cream to the mixing bowl and beat on medium for about 2 minutes.
Now here's where you become "flexible" and add .5 to 4.5 cups of icing sugar to the mixture to increase volume and sweetness to your liking.
We do this about .5 c at a time.
Accordingly, add milk or cream by the scant tablespoon to achieve your desired spreading or piping consistency.
We always end up making more icing than we need, using about 6 tbs total milk and 7-7.5 c icing sugar, but this varies by taste, temperature, and the size of the cake/cupcakes you are frosting.
Warning, this "flexible" recipe may require a fair bit of tasting and, hence, result in a bit of a "sugar high" pour le chef.