Royal icing is primarily composed of which two ingredients?

Royal icing is primarily composed of egg whites and confectioners' sugar, which is why this answer is correct. The use of egg whites provides the necessary structure and stability to the icing, allowing it to dry hard and create a smooth finish that is ideal for decorating cookies and cakes. Confectioners' sugar, being finely ground, dissolves easily, contributing to a sweet flavor and a glossy texture. This combination is especially valued for its ability to hold intricate piping designs and withstand handling without losing its shape.

The other options include ingredients that are not typically used in royal icing. Flour does not play a role in royal icing, as this icing is meant to be a smooth, pourable substance rather than a batter. Butter and sugar are components of other types of icing, such as buttercream, where butter provides richness and a creamy texture. Heavy cream and vanilla are flavorful additions often used in frosting or whipped creams, but they do not contribute to the hardening properties that define royal icing.