To make pasta:
1. Combine cocoa and hot water to make a paste. Combine cocoa paste, bittersweet chocolate, semolina flour, sugar, 2 eggs in a mixing bowl with dough hook. Mix dough until smooth, remove from bowl and knead adding flour as necessary until dough is no longer sticky. Wrap in plastic and refrigerate for 1 hour.
2. Pass dough through a pasta machine according to manufacturer’s instructions to make 2 sheets measuring 6-inches by 15-inches.
3. Cut pasta sheet into 3-inch squares. Place 1 tablespoon of Nutella spread in the center of each square. Lightly beat 1 egg and brush the edges of the pasta squares, then fold diagonally over the filling, pressing gently to seal.
4. Preheat the oven to 200°F. Add enough oil to a heavy large frying pan to reach a depth of 2 inches. Heat the oil over medium heat to 350°F.
5. Working in batches, carefully add the ravioli to the hot oil and cook until they are golden brown, about 45 seconds per side. Using a slotted spoon, transfer the ravioli to a plate lined with paper towels to drain. Then, transfer the cooked ravioli to another baking sheet and keep them warm in the oven while frying the remaining ravioli.
For mascarpone cream:
1. Mix 1 container of mascarpone (500 g) with ¼ cup granulated sugar and ½ teaspoon vanilla extract.
1. Place a dollop of the mascarpone cream in the center of a plate, surrounded by 5 ravioli; sprinkle with toasted hazelnuts and serve.