Whisk together 2 tablespoons of the instant espresso granules and 2 cups of the boiling water in a bowl until coffee is dissolved. Let cool for 1 hour. Stir in liqueur, if desired, and ¼ cup of the milk.
Pour into a 12-cup Bundt pan, and freeze until frozen, at least 6 hours or up to 12 hours.
Remove Bundt pan from freezer, and run outside of pan under warm water to unmold ice ring from pan. Place ice ring on a freezer-safe plate, and place in freezer.
Whisk together sugar and remaining 1 cup boiling water and 4 tablespoons instant espresso in a large bowl until sugar dissolves, about 1 minute.
Stir in remaining 6 cups milk until combined.
Drizzle chocolate syrup all around the inside of 5-qt. punch bowl (or larger).
Add milk mixture and ice cream slices to bowl. Stir carefully until ice cream is just melted.
Add ice ring to bowl, and garnish with chocolate shavings.
