Stiff icing -- also known as royal icing -- holds its form after application, which makes it useful for creating standing figures or as glue to hold together gingerbread houses or cake structures. Royal icing uses egg whites and hardens when exposed to air, so it's best to move quickly when decorating with it. Substituting vegetable shortening and water for the egg whites produces another stiff icing called buttercream. The amount of icing sugar used has the biggest impact on the consistency of the icing, so adjust your measurements accordingly.
Beat the egg whites with the lemon juice until foamy in the bowl of a stand mixer or with a hand mixer. For the buttercream alternative, combine the shortening with the extract and water using a mixer.
Turn the mixer to low speed and sprinkle in the sifted icing sugar 125 ml (1/2 cup) at a time. Continue to beat the mixture until each cup of icing sugar is incorporated fully. Stop adding the icing sugar when the icing becomes difficult to beat and forms stiff peaks when you lift the beaters out of the mixture.
Use the royal icing immediately or press a piece of cling film against the surface to prevent hardening. Store buttercream icing in the fridge indefinitely in an airtight container.