American buttercream, also known as crusting buttercream, is a perfectly stiff buttercream that crusts over so that when you touch it, it's no longer sticky.
Place shortening and butter in a stand-mixer with a paddle attachment and cream together until blended and smooth. Scrape bottom of bowl often.
Place some plastic wrap over the mixer to keep some of the powdered sugar from escaping while you mix
Add all the sugar one cup at a time on low until incorporated. Add vanilla, salt and milk and mix on low until blended and smooth. Do not turn up the mixer or you will get air bubbles.
Let it mix on low for 10 minutes so it's perfectly smooth.
Cover bowl with plastic wrap until you're ready to use it. Store in air-tight container in the fridge or freeze it. Bring to room temperature and mix on low before each use.
OPTIONAL: Make two batches separately. After the second batch is done, take some buttercream from the first batch and add it to the bowl mixing on low speed until the paddle is covered. Let mix on low with the paddle attachment for 10 minutes until there are no air pockets and the texture is smooth. (instruction adapted from Swank Cake Design's Crusting Buttercream recipe)