Start by placing your softened butter in a large mixing bowl. Beat the butter on medium speed with an electric mixer until it becomes light, fluffy, and very creamy—this usually takes about 2 to 3 minutes.
Gradually add in the powdered sugar, 1 cup at a time, continuing to beat the mixture on low speed.
Now it’s time to incorporate the star ingredient: sweetened condensed milk. Add it one tablespoon at a time, mixing on low speed until each addition is fully integrated.
If your frosting is too thick, feel free to add a little more sweetened condensed milk, or if it's too sweet, adjust by mixing in a bit more lemon juice.
For an extra zesty finish, stir in the optional fresh lemon zest with a spatula.