Prepare the oil: Pour about 2 inches of vegetable oil into a deep, heavy-bottomed pot or Dutch oven. Heat the oil over medium heat until it reaches 350 degrees Fahrenheit. Use a thermometer to check the temperature.
Prepare the filling: Spoon the Nutella into a piping bag fitted with a long, thin tip, or use a small zip-top bag with a corner snipped off. Set aside.
Prepare the coating: In a shallow dish, mix the granulated sugar and ground cinnamon together.
Shape the donuts: Open the biscuit dough. Gently flatten each biscuit slightly. Place about 1 teaspoon of Nutella in the center of the flattened dough. Fold the edges up and over the filling, pinching the seams tightly to seal the Nutella inside. Roll the dough gently between your palms to form a smooth ball.
Fry the donuts: Carefully drop 2 or 3 filled donuts into the hot oil, ensuring you do not overcrowd the pot. Fry for about 1 to 2 minutes per side, turning them gently with tongs until they are golden brown.
Drain and coat: Remove the cooked donuts with a slotted spoon and place them on a plate lined with paper towels to drain excess oil. While still warm, roll each donut in the cinnamon-sugar mixture.
Serve: Serve the donuts immediately while the Nutella center is molten. Dust with powdered sugar before serving.
Notes
If you do not have a piping bag, you can freeze small dollops of Nutella on parchment paper for 15 minutes before filling the dough.
Maintain the oil temperature between 340 and 360 degrees Fahrenheit for best results. If the oil is too cool, the donuts will absorb too much grease.