Combine the chocolate and flour in a medium saucepan.
Blend in the milk.
Cook over medium heat, stirring constantly, until very thick.
Remove from heat and cool.
In a large bowl, cream the butter, sugar and vanilla.
Beat until light and fluffy, about 3 minutes.
Gradually add the cooled chocolate mixture, beating after each addition.
Beat on high speed until frosting is a whipped cream consistency.