ISO: Need help adapting whipped icing

My family loves the "whipped cream" icing I picked up from this board. It's the kind that starts with milk and flour cooked together, and when that has thickened and cooled, butter and sugar are whipped in. Other than having to press it thru a strainer every time, I think it's a great recipe. Now I'd like to adapt it to make a chocolate icing. Can anyone tell me whether and when I can add either cocoa or melted chocolate? I'd like to put this on that bday cake I'm working on. But I have more birthdays coming up in November, so whenever anyone has some coments, I'll be happy to read them! Thanks for any advice.