Beat the butter until creamy in your electric mixer and then beat in the sugar until light and fluffy, about 2 to 3 minutes. Beat in the eggs one at a time until combined. Beat in the lemon juice - if it looks a little curdled, that's okay! Transfer the mixture to a saucepan and heat over low heat. Cook until thickened, stirring the whole time, about 10 minutes. Remove and let the mixture cool completely, then refrigerate it.