In the bowl or a stand mixer, or in a large bowl if using a hand mixer, cream the butter on high until smooth and fluffy–about 3-5 minutes. Add 1 cup of powdered sugar and mix on low until the sugar begins to combine with the butter, then turn the mixer up to high. The buttercream might be a bit dry and crumbly at this point. Add in the orange zest and the juice of half an orange and beat on high until fully combined and the frosting begins to come together. Slowly add in the remaining two cups of powdered sugar, one at a time, starting on low and increasing the speed as the powdered sugar begins to mix in to the buttercream. Add in the salt. Beat on high about 3-5 minutes until light and fluffy. If your buttercream is a bit stiff, add in 1-2 Tbsp of milk, one at a time, until you have a smooth and pipeable consistency.