The Lemon Buttercream Icing by Wilton is a good homemade lemon icing recipe. The recipe makes 3 cups of icing, and it has a rating of 4 1/2 out of 5 stars from more than 30 user ratings on Wilton.com, as of 2015.
To prepare the recipe, cream 1/2 cup solid vegetable shortening and 1/2 cup softened butter together in a large bowl using an electric mixer, until light and fluffy. Add 2 tablespoons lemon juice and 1 teaspoon lemon zest to the icing, beating thoroughly. While beating the icing on medium speed, add 4 cups of powdered sugar to the mixture, 1 cup at a time. For optimal mixing, stop the beater frequently to scrape the sides and bottom of the bowl. If the mixture is dry after adding all the sugar, beat in a little milk or lemon juice, depending on the level of tartness you desire, until it reaches a light and fluffy consistency.
Cover the icing with a damp cloth, and place it in the fridge until you are ready to use it, and then whip the mixture once more before using it. The icing combines well with a variety of desserts, including lemon cake, margarita cupcakes, curd-filled cupcakes and white-grape juice cupcakes.