2) While the pineapples are cooking, whisk the mascarpone and vanilla in a small bowl to blend. Set aside.
3) Combine the chocolate-hazelnut spread and cream in a small bowl and stir to combine. Place in a microwave on high for about two minutes, stirring every 20 seconds to blend well. Alternatively, you can combine the chocolate-hazelnut spread and cream in a small saucepan, set over low heat, and cook for about 5 minutes, stirring constantly. Heat until satiny smooth and easy to pour, about 3 to 5 minutes.
4) Transfer the pineapple slices to a large platter. Drizzle the warm chocolate-hazelnut sauce. Dollop the mascarpone mixture atop. Sprinkle with the hazelnuts and serve.