Whisk together sugar, flour and cocoa powder in a saucepan. Add evaporated milk and butter. Bring mixture to a low boil over medium heat, stirring frequently. Gently boil mixture for 3-4 minutes until thickened. Allow mixture to cool about 5 - 10 minutes before serving. Whisk once more before serving.
Store unused portion in the refrigerator up to 2 weeks, reheat in the microwave until warm.