In a stand mixer:

Cut butter into about 5 pieces per stick, and add to bowl of mixer. Add icing sugar, cocoa powder, and salt; blend on low. Slowly add the golden syrup while mixer is going on low, then add the water. After it's all added, turn it up to mix it until it's incorporated, then add the vanilla and the melted chocolate. Mix well until completely blended. You can make this up to 3 hours in advance. For longer storage, refrigerate the frosting, covered, and let it stand at room temperature for 1 hour before using.

In a Food Processor:

In a food processor, process butter, sugar, cocoa, and salt until smooth, about 30 seconds. Scrape the bowl as needed. Add the syrup and vanilla and process until just combined, about 5-10 seconds. Scrape down the bowl, and then add the chocolate and pulse until smooth and creamy, about 10-15 seconds. The big key to this recipe is using a light hand with the processor and not over mixing it. You can make this up to 3 hours in advance. For longer storage, refrigerate the frosting, covered, and let it stand at room temperature for 1 hour before using.