In a small heavy saucepan melt chocolate chips and butter, stirring frequently.
Stir in sugar; gradually stir in milk.
Bring to a boil over medium-high heat; reduce heat to low.
Boil gently for 8 minutes, stirring frequently.
Remove from heat.
Serve warm.
*Note: Store leftover sauce in the refrigerator.
Reheat in a microwave safe dish on high for 15 to 20 seconds.