Combine cream, butter, and corn syrup in a saucepan over medium-high heat; bring to a boil. Cook 1 minute or until sugar dissolves. Remove from heat. Add 1 2/3 cups chocolate and liquor (do not stir); let stand 1 minute. Gently swirl pan until smooth. Pour into a bowl; chill, stirring every 15 minutes, until thick enough to hold shape (about 45 minutes). Using 2 spoons, shape the mixture into 1-inch balls (about 2 teaspoons each) and place on wax paperlined plates. Refrigerate until chilled (15 minutes).
Meanwhile, in a bowl set over a medium saucepan of simmering water, gently melt remaining 2/3 cup chocolate. Pour melted chocolate into one bowl and chopped pistachios and orange zest in another bowl. Dip each chilled truffle into chocolate first, then into pistachio-orange mixture, rolling to coat. Refrigerate until firm (about 8 minutes) or until ready to serve.