To create a shell border for cakes, which tool is required?

A pastry bag with a plain tube is essential for creating a shell border on cakes. This tool allows for precise control over the piping of the icing, which is crucial for achieving the desired shape and consistency of the shell border. The plain tube helps to create a clean and uniform design, as it can be used to apply even pressure while squeezing the bag, resulting in a consistent and professional-looking shell pattern around the edges of the cake.

While other tools like a pastry brush can help apply glazes or washes, and a spatula is useful for spreading frosting or smoothing surfaces, they are not designed for piping intricate decorations. Cookie cutters serve a completely different purpose, as they are used for shaping dough rather than applying icing or creating borders. Thus, the pastry bag with a plain tube stands out as the necessary tool for this specific technique.