In a bowl, combine tapioca pearls with water to cover. Stir well and set aside for 20 minutes. Drain, discarding liquid. Transfer to prepared baking dish.
Meanwhile, in a saucepan, heat 1 cup of the milk over low heat just until simmering. (Do not boil.) Remove from heat. Add chocolate and stir until melted. Pour into prepared baking dish and stir to combine.
In blender, combine eggs, sugar, remaining 1 1/2 cups of the milk, liqueur and vanilla. Blend until smooth. Add to prepared baking dish and stir well. Cover with foil and secure with string. Place dish in slow cooker stoneware and add enough boiling water to come 1 inch up the sides of dish. Cover and cook on High for 2 1/2 to 3 hours, until a knife inserted in the pudding comes out clean. Stir well. Serve warm or cover and chill overnight. Top with a dollop of whipped cream. Serves 6.