To make the chocolate sauce, add sugar, corn syrup, milk, cocoa powder, espresso powder, and salt in a sauce pan over medium heat. Bring to a simmer.
Once the sauce is simmering, add 1/2 tsp of vanilla extract and turn off the heat.
When the heat is off, add the chopped bittersweet chocolate and mix until smooth. Put in a jar or bowl to cool. You can save the chocolate sauce for a later use or wait 30 minutes for it to cool for the shake.
Add the milk, 1 tsp of vanilla extract and vanilla ice cream to a blender. Blend until smooth and to the consistency that you like. Add more milk if you like a less thick shake.
Take a glass and layer the shake and chocolate sauce. Top with whipped cream (optional) and enjoy!