In a bowl, whisk together the heavy cream and instant espresso gently until the espresso is roughly 95% dissolved.
Whisk in the sweetened condensed milk, chocolate syrup, and vanilla extract just until blended.
Add the whiskey in 1/3 cup increments, whisking gently after each addition until fully incorporated.
Pour into an empty whiskey bottle, large mason jar, or other storage bottle.
Refrigerate; it will keep for at least 3 weeks (and has stayed perfect for up to 6 weeks).
Shake well before pouring.