In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium-high speed for 6-7 minutes.
With the mixer on low speed, slowly add in the salt and powdered sugar, and continue beating until the sugar is fully incorporated.
Add in lemon juice, lemon zest, and milk, and mix on low speed until incorporated.
Turn the mixer back up to medium-high speed and beat the frosting for an additional 6-7 minutes.
If the frosting is too thick, add in a bit of milk, one teaspoon at a time until you reach the desired consistency.
Notes
You can easily adjust the consistency of this recipe by simply adding in more milk a splash at a time.
If you are a fan of shortening in your buttercream, simply omit one stick (1/2 cup) of butter and replace with 1/2 cup shortening.
Covered and refrigerated frosting can be stored for up to three days. Simply bring it to room temperature and beat with an electric mixer until smooth. You may need to add a couple of teaspoons of heavy cream or milk to revive the consistency. Frosting can also be frozen for up to a month.
If you garnish with sprinkles, make sure you do this immediately after you frost your baked goods. Once the top layer of the buttercream crusts, nothing will stick.