For the syrup: combine 350ml water, the caster sugar, McDougalls Cocoa Powder and salt in a saucepan over a medium heat. Bring to a simmer until the syrup starts to thicken up.
Remove from heat and stir in the vanilla extract. Allow to cool and keep refrigerated until used.
For the martini: pour the chocolate syrup, Irish cream and chocolate liquor into a cocktail shaker with the ice and shake until blended.
Pour each martini into a cocktail glass and garnish with shaved chocolate.