The single serve recipe on the Hershey's cocoa box is good though a bit strong. There is also a 6 serving one.
Single: 2 Tbsp sugar, 2 Tbsp cocoa, (dash salt can be omitted) 1 cup milk, 1/4 teasp vanilla. Sometimes use soy milk and vanilla ice cream can be substituted for part of the milk. Add any flavoring.