Add the milk, sugar and salt in a pan, put on a medium heat, and stir until the sugar has completely dissolved. Cook for about 30-40 minutes, stirring every few minutes. You are looking for a consistency where it is a bit thicker than cream, and when it just seems to coat the spoon. Take the pan off the heat and add the chocolate, and stir until it is completely melted and incorporated into the milk. Let it cool completely in the fridge (it will thicken up a lot more when cooled).
When cooled add to a mixing bowl and whisk for a few seconds, then add cream and whisk it together for a few minutes until it thickens a bit.
Add the other ingredients and whisk until the instant coffee powder has dissolved and the consistency is thick and creamy like Baileys.
Run the mixture through a sieve and pour into a sterilised bottle. Store in the fridge. It will keep for about 1-2 weeks, but give it a good shake before serving.