Place chocolate in a large heatproof bowl. In a small, heavy saucepan, bring cream to a boil and pour over chocolate, whisking until melted and smooth. Cool slightly. Whisk in chocolate-hazelnut spread until combined. Let cool completely, then refrigerate until firm, at least 3 hours or overnight.
Using a teaspoon, scoop out a 1- to 1 1/2 -inch nugget. Use your hands to quickly roll it into a ball; transfer to a rimmed baking sheet. Continue with remaining mixture. (If balls become too soft, refrigerate for a few minutes.) If using, roll truffles in cocoa to coat. Gently press a sugar decoration or toasted hazelnut into top of each truffle, if desired. Cover and refrigerate until firm. (Refrigerate unadorned truffles in an airtight container for up to 2 weeks. Bring to room temperature before decorating with candy or nuts. Refrigerate truffles again before serving.)