Heat chocolate-hazelnut spread in a metal bowl over a saucepan of gently simmering water and stir until warm, smooth, and easily stirred, about 5 minutes. Remove the bowl from heat.

Dissolve sugar in 2 tablespoons water in a small saucepan over medium heat. Brush the sides of the saucepan with a moistened pastry brush occasionally as mixture cooks. Increase heat to medium-high, bring syrup to a boil, and cook until syrup is a deep amber color, brushing down the sides and swirling the saucepan occasionally to prevent scorching, about 4 minutes.

Pour in cream while stirring. Reduce heat to low and cook, stirring continually, until a smooth caramel develops, 5 to 10 minutes. Gently stir into melted hazelnut spread with 1/4 teaspoon sea salt. Refrigerate caramel mixture until firm, at least 3 hours.

Spread cocoa powder into the bottom of a wide, shallow bowl.

Scoop caramel mixture with a melon baller into 1 tablespoon portions, then roll in cocoa powder to coat and arrange onto a baking sheet. Cover truffle balls with plastic wrap and chill in the refrigerator, 8 hours to overnight.

Line a 13x9x2-inch baking sheet with aluminum foil.

Heat chopped chocolate in a metal bowl set over a saucepan of gently simmering water, stirring frequently until chocolate is melted and smooth and reaches 115 degrees F (46 degrees C). Remove the bowl from heat.

Working quickly, submerge each chilled truffle ball in melted chocolate. Lift truffles from chocolate with a fork and tap the fork against the side of the bowl to remove excess coating. Transfer truffle to the prepared baking sheet. Repeat with remaining truffle balls.