In a medium saucepan over medium-high heat, whisk the flour and milk until thickened. Once it is thickened, set it aside and allow to cool.

In a large bowl, cream the butter, sugar, and vanilla with an electric mixer.

Add the flour and milk mixture and continue to beat with the mixer until the icing is light and fluffy.

Add the cacao or a food coloring of your choice part way through the mixing process.