In a stand mixer, add butters and shortening and cream together at medium speed. Add clear vanilla and mix well.

Add powdered sugar in batches, in halves or thirds and mix well at low speeds. Be sure to cover the mixer with a clean towel to ensure the powdered sugar does not go everywhere. Scrape down the sides with a rubber spatula.

Add milk while mixer is still on (start at a low speed), one tablespoon at a time until you reach desired thickness. In between adding, you will need to scrape down the sides. Once you have the milk mixed in, mix at medium speed for 3 to 4 minutes. For thinner icing, use more milk. For thicker icing, use less milk.