1. Preheat grill for medium heat and lightly oil the grate.
2. Heat chocolate-hazelnut spread and milk in a small saucepan over medium heat, stirring until spread is melted, about 5 minutes. Remove from heat.
3. Grill pineapple on preheated grill until lightly browned, 3 to 5 minutes per side. Transfer pineapple to a platter. Drizzle with chocolate mixture and sprinkle walnuts on top.