"Dark chocolate, hazelnut, and caramel were just made to be eaten together. How can you resist the indulgent blend of sweet, nutty, with a dash of salt? It's perfect. They can be made up to a week before you want them, just cover and chill. Bring to room temperature before serving."
Heat hazelnut spread in metal bowl over saucepan of gently simmering water and stir until hazelnut spread is warm, smooth, and easily stirred, about 5 minutes. Remove bowl from heat.
Dissolve sugar in water in a small saucepan over medium heat. Brush sides of pan with a moistened pastry brush occasionally as sugar mixture cooks. Increase heat to medium-high and bring syrup to a boil and cook until syrup is a deep amber color, brushing down sides and swirling saucepan occasionally to prevent scorching, about 4 minutes.
Pour cream into the syrup while stirring. Reduce heat to low and cook, stirring continually, until you have a smooth caramel, 5 to 10 minutes; gently stir into the 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; roll in the cocoa powder to coat; arrange onto a baking sheet.
Cover truffle balls with plastic wrap and chill overnight or 8 hours.
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 the chocolate reaches a temperature of 115 degrees F (46 degrees c). Remove bowl from over water.
Working quickly, submerge each truffle ball in the melted chocolate. Lift truffles from chocolate with a fork and tap fork against side of bowl to remove excess coating. Transfer truffle to foil-lined pan to cool. Repeat with remaining truffle balls.
Sprinkle finished truffles lightly with 1 tablespoon coarse sea salt before the chocolate hardens. Let stand until coating sets completely, at least 1 hour.
When shaping the truffle filling, try to avoid touching it as much as you can. I use a melon baller to do the majority of the shaping because the moment you touch it, it starts to melt. If the mix gets too soft, let it chill in the freezer for 45 minutes to 1 hour before continuing.