In a stand mixer with the paddle attachment, cream shortening and butter on medium-high until fully combined and fluffy, about 3 minutes.
With mixer on low speed, add lemon juice (or extracts) and salt.
Slowly add sugar, one cup at a time. Add meringue powder. (The mixture will appear dry.)
Add in milk. Turn the mixer up to medium-high (or high) and beat until light and fluffy (approximately 5 to 8 minutes).
*You may need to add more milk to reach your desired consistency. Start with the 1/4 cup and if the buttercream is still too dry, add in more milk 1 tablespoon at a time until you reach the desired consistency. Make sure the mixer is on low speed when adding in milk and give it at least 30 seconds between each addition.
**You can omit meringue power if you prefer. To achieve the perfect piping consistency, you may need to add more confectioner's sugar.
For extra smooth frosting with less air pockets, add buttercream to a food processor 1/2 batch at a time. Process for 3-5 minutes on high, or until frosting appears smooth and creamy like ice cream.
Place directly into piping bags and seal with a rubber band. Can be stored in piping bag until ready to use, up to 3 months.