Gradually stir in enough lemon juice to make a thick, soft peak consistency. When the mixture is lifted with a spoon, the peaks formed should fold over and barely hold their shape.
½ teaspoon lemon juice
Adjust the consistency with more sugar if needed.
Add the icing to a piping bag fitted with a #2 tip and make your desired designs on the cooled cookies.
Allow the icing to dry before enjoying.
Notes
Some of the granulated sugar can be substituted with brown sugar to further deepen the flavour. However, this can alter the moisture content of the cookie, so the water may need to be reduced.
The icing can be dyed. Use vegan food colouring to create as many colours as desired.
Be sure the cookies have cooled for 2 minutes before moving off the baking sheets. They can crack or fall apart if moved before they have cooled enough.
Using aquafaba in royal icing helps it dry smooth, matte and hold its shape.
Do not decorate cookies until they have fully cooled, or the icing will melt.
Store for up to a week in a tightly sealed container. Be sure to keep them as dry as possible, as humidity can cause the icing to soften.