Preheat your sous vide circulator to 167°F.
Combine the milk and cream in a saucepan and heat to a low simmer, around 167°F (75°C). Remove from the heat.
Whisk the egg yolks, sugar and salt until combined.
Temper your egg mixture with a little bit of the warm milk, then add the eggs into the cream. Add the chocolate and whisk until melted.
Use an immersion blender to make sure all the ingredients are thoroughly combined.
Pour the mix into 4 or 8 oz mason jars and screw on lids finger tight (if desired, you can remove the bubbles on top with a spoon or with low heat from a torch).
Place into the sous vide bath for 1 hour. Once finished, remove into an ice bath to cool, then put in the fridge overnight to chill fully.
Place all ingredients into a saucepan
Boil over medium high heat until it reduces to 1/2 to 1 cup of syrup.
Strain through a fine mesh strainer and place in a mason jar.
Allow it to come to room temperature on the counter, then refrigerate until ready to serve (can be made 1-2 days in advance)