In a saucepan over medium heat, whisk together the milk and flour until smooth. Cook until thickened, about 5-7 minutes. Remove from heat and let cool.
In a mixing bowl, cream the softened butter and sugar until light and fluffy.
Once the milk mixture is cooled, add it to the butter mixture along with vanilla, lemon juice, and zest. Beat until well combined and fluffy.
Use immediately or store in the fridge until ready to use.
Notes
This frosting pairs beautifully with lemon-flavored cakes.