Versatility is the name of the game with this icing. Substituting liqueur, milk, or another citrus juice for the lemon juice lets you customize the icing for a variety of cakes. It's especially delicious drizzled atop loaf cakes.
Whisk together sugar and lemon juice until smooth. If icing is too thick, add more lemon juice, 1 teaspoon at a time. Use immediately.