Hot Fudge Milkshake

Name: Hot Fudge Milkshake
Ingredients: For hot fudge:; ¼ cup Dutch-process cocoa powder (unsweetened); ¼ cup packed dark brown sugar; ½ cup golden syrup; 2/3 cup heavy cream; 6 oz good quality semi-sweet chocolate, finely chopped; 2 T butter (unsalted), cut into small pieces; 2 tsp vanilla extract; Pinch of sea salt; To assemble hot fudge shakes:; ½ cup to ¾ cup whole milk; 2 ½ cups good quality vanilla ice cream; ¼ cup hot fudge, cooled if freshly made; Maraschino cherries and freshly whipped cream (optional)