Place sugar and salt in a medium saucepan. Sift cocoa powder and flour over top and whisk to combine. Whisk in boiling water, golden syrup and butter.Bring to a boil over medium heat, stirring occasionally; boil for 8 to 10 minutes or until thickened slightly and reduced by about a third.Remove from heat and transfer to a heatproof bowl; stir in vanilla.Let cool to room temperature, stirring occasionally, for about 30 minutes (don’t refrigerate the sauce until it has completely cooled otherwise it may become grainy).Transfer cooled sauce to a glass jar or airtight container and refrigerate for up to 2 weeks. Warm gently in the microwave before serving.