In a bowl, mix the cocoa powder with 3 tablespoons warm milk until smooth.
In a small, thick bottomed saucepan, heat the remaining 5 tablespoons milk with the flour. Stir well to get rid of any lumps.
Add the sugar, stirring constantly with a wooden spoon. Once the sugar has dissolved, reduce heat and add the butter. When butter melts, add the cocoa mixture. Simmer 5-10 minutes on low heat until thick.