A classic icing recipe I was given in cake decorating class
Adding all the confectioners' sugar at once creates a dense, grainy paste that refuses to spread smoothly. Fat must be thoroughly aerated before sugar enters the bowl. Creaming the shortening and butter together on medium speed for four full minutes incorporates the necessary micro-bubbles required for a light texture.
Visual cues dictate when the mixture reaches the correct stage. The fats should transform from a pale yellow into a nearly white, fluffy mass that holds distinct ridges from the mixer attachments. Introduce the liquid elements gradually after this aeration phase occurs. Incorporating the sugar one cup at a time prevents the powdered sweetener from flying out of the bowl and ensures complete dissolution without overworking the gluten-free structure.
Assessing consistency requires a spatula test. Draw a clean line through the center of the bowl contents; the sides should slowly slump inward while retaining soft peaks. If the mixture appears too stiff, incorporate an additional half teaspoon of milk. Scaling this formulation up or down requires maintaining the exact one-to-one ratio between solid fats and liquid components to preserve structural integrity.