In a small saucepan, combine sugar and cocoa powder. Whisk in evaporated milk.
Add butter and cook and stir, stirring constantly, over medium-high heat until mixture begins to boil.
Remove from heat and stir in vanilla extract.
Serve immediately or store in the refrigerator for later. For best results, reheat in the microwave or on the stovetop before drizzling over your favorite desserts.
Notes
Substitute almond, raspberry, or orange extract in place of vanilla for different flavored chocolate sauce.