In a heavy saucepan whisk together sugar cornstarch, and a pinch salt. Chop chocolate and add to sugar mixture. In a bowl whisk together milk and egg yolks and gradually whisk into chocolate mixture. Bring mixture just to boil over moderate heat, whisking constantly, and boil 1 minute, whisking. Remove pan from heat and whisk in butter, vanilla, 1-ounce cacao nibs, and1 tablespoon Kahlua. Divide pudding between 4 (8-ounce) ramekins. Chill puddings in freezer, surfaces covered with plastic wrap, until cooled, about 30 minutes. Garnish with whipped cream and crystallized ginger.