In a heatproof bowl set over a pan of simmering water, melt the chocolate and butter. Set aside to cool slightly then add the melted chocolate mixture to a bowl of a stand mixer fitted with a whisk attachment.
Add the golden syrup and vanilla extract to the chocolate mixture and mix briefly.
Sift half of the icing sugar over the frosting mixture and whisk to combine.
Add the sour cream and beat until the frosting is well combined and homogeneous. At first, the mixture will look lumpy, just continue beating at a medium-high speed.
Sift over the remaining icing sugar and beat at medium-high speed until the frosting is fluffy.
With the mixer's motor running at medium-high speed, add the cream and salt and beat until fluffy.
Add the frosting to a piping bag or use an offset spatula to smooth it over cakes, cupcakes, or cookies.