Easily as quick and simple as a package, and no preservatives! 

6 tbsp cornstarch

3 cups milk

4 tbsp cocoa powder

1 cup icing sugar

1/2 cup butter

Combine cornstarch, cocoa powder and icing sugar- mix well.  Stir in milk.

Stir dry ingredients together and whisk in milk.  Cook over medium heat until mixture comes to a gentle boil and continue to boil – stirring so it doesn’t stick or boil over.  When it has thickened a bit, remove from heat and stir in the butter until melted. 

Easily adapted to the microwave – combine dry ingredients and whisk in milk – heat on high power stirring every couple of moments until it boils and thickens – remove from microwave and stir in butter.