This frosting is the perfect combination of rich, fluffy and bright. It has a lovely lemon flavor without being too much and the sweetened condensed milk adds that little something special you can't quite put your finger on!
Slowly mix in the 4 cups powdered sugar, 1 teaspoon lemon extract, and 1-2 Tablespoons lemon juice until combined.
Add 4-5 Tablespoons sweetened condensed milk a tablespoon at a time until you reach your desired consistency.
Adjust with a bit more sweetened condensed milk or lemon juice to loosen it up or powdered sugar if it needs to be stiffer.
Stir in finely grated zest of a lemon if desired. Use to frost cooled cake.
Notes
You can store leftover buttercream in the refrigerator for a week or two. Or put it in the freezer for up to three months. Just bring it back to room temperature and mix well before using.
The recipe on its own will be a very pale yellow from the butter and lemon zest. For a more vibrant lemony yellow, add a drop or two of yellow food coloring gel. Start slow and add until you get just enough. The color will deepen slightly as it sits.