This is the recipe my mom has always used. It's easy and delicious!
Start with 2 sticks of room temperature butter in a stand mixer.
Turn on the mixer and add 2 tsp. of vanilla.
Add 1/4 tsp. of salt (give or take a pinch... I just sprinkle a bit in)
Turn the mixer down to it's lowest speed and add a whole bag of confectioners sugar (2lbs).
Start adding milk slowly. I use between 1/3 and 1/2 of a cup. Be careful not to add the full 1/2 of a cup until you scrape down the sides of the bowl and check to see if you really need it. You can always add more if it's too stiff but you can't take it out if you've added too much (but you can add more sugar if you're stuck with a runny icing)
Turn up the mixer to medium speed and let it do it's thing. Scrape down the sides frequently. I let it blend for a couple minutes then check the consistency of the icing. It should be thick but not firm. You want to be able to easily spread it but it needs to hold it's shape and not slide off the cake. This is enough to frost a 9X13 sheet cake or a 8" or 9" layer cake.
**If you want chocolate icing, add 1/2 cup of cocoa powder with the powdered sugar. You may have to add a tiny bit more milk. **
