Combine butter, both types of sugar, milk, salt in a saucepan and bring to a boil;
Cook for 6 minutes on low heat, stirring constantly;
Cool mixture slightly, add vanilla, cocoa, and confectionery sugar;
Mix thoroughly until smooth;
Pour the fudge onto a baking sheet (or silicone mat) and cool;
Form a log, wrap in plastic wrap and refrigerate for half an hour;