Chill your martini glass by placing it in the freezer or filling it with ice water.
Add whiskey, chocolate liqueur, half-and-half, simple syrup, and Irish cream (if using) to a cocktail shaker.
Fill the shaker with ice and shake vigorously for 20 seconds until well chilled and slightly frothy.
Discard the ice water from your martini glass and swirl chocolate syrup inside the glass or dip the rim as desired.
Strain the cocktail into the prepared martini glass.
Garnish with chocolate shavings, a dusting of cocoa powder, or white chocolate curls.
Serve immediately and enjoy.