Put the milk in a saucepan and whisk in the flour. Bring to a simmer over medium heat and simmer until thickened, about 5 minutes, whisking often. Strain and cool to room temperature.
Cream the butter, sugar, and vanilla in a mixer fitted with a paddle attachment until light and very fluffy. Add the cooled thickened milk and mix until smooth. Drizzle in the melted chocolate and continue whipping until well blended.
Recipe courtesy of Gale Gand