In a large mixing bowl using an electric mixer on low speed cream the butter.
Add cocoa powder, vanilla and chocolate syrup. Make sure to mix well to prevent lumps.
Gradually add the powdered sugar, one cup at a time. Scrape down the sides and bottom of the bowl. Mix for 1-2 minutes, until smooth and fluffy.
If the frosting seems stiff, add the whipping cream and blend for another minute.