Start by placing 8 ounces of semi-sweet chocolate in a large heatproof bowl. Microwave it in four fifteen-second increments, stirring after each interval, for a total of 60 seconds. Stir until the chocolate is almost completely melted, then set it aside.

In a small saucepan over medium heat, pour in the heavy cream. Whisking occasionally, heat the cream until it just begins to boil, then remove it from the heat and pour it over the melted chocolate.

Gently stir the chocolate and cream mixture with a wooden spoon in one direction until smooth. Add the Nutella and softened butter, blending until everything is combined. Stir in 1/2 cup of the finely chopped hazelnuts.

Cover the bowl with plastic wrap pressed against the chocolate mixture and let it sit for 15-30 minutes at room temperature. Then, transfer it to the refrigerator until it firms up, which takes about 4 hours.

Line a large baking sheet with parchment paper or a silicone baking mat. Remove the chilled mixture from the refrigerator and measure out 1 teaspoon of the mixture. Roll it into a ball and place it on the baking sheet.

Melt the remaining 8 ounces of chocolate in a double boiler or microwave, stirring until completely melted and smooth.

Using a dipping tool or fork, dip each truffle into the melted chocolate. Tap the tool gently on the bowl's side to remove excess chocolate and place dipped truffles back on the baking sheet.

Drizzle any leftover chocolate over the truffles and sprinkle the remaining hazelnuts on top.

Refrigerate the truffles for at least 30 minutes until the chocolate has fully set before serving.