To make the syrup, combine all the ingredients in a saucepan and allow to come to a simmer. Cook until the syrup has reduced and thickened. Remove from the heat and allow to cool.
To make the milkshake, combine all the ingredients in a blender and blend until smooth.
Swirl the coffee syrup into chilled glasses. Pour in the milkshake and top with whipped cream and more syrup. Serve immediately.