Line an 8x8-inch square baking pan with parchment paper, leaving a 2-inch overhang on two sides for easy removal. Lightly grease the parchment with butter or cooking spray.

In a large microwave-safe bowl, combine the chocolate chips, sweetened condensed milk, chocolate hazelnut spread, and butter. Microwave in 30-second intervals, stirring between each, until completely smooth and glossy, about 2-3 minutes total.

Stir in the vanilla extract and sea salt until fully incorporated. The mixture should be thick and velvety.

Fold in the toasted chopped hazelnuts, reserving about 2 tablespoons for topping if desired. Mix until the nuts are evenly distributed throughout.

Pour the fudge mixture into the prepared pan and spread into an even layer using an offset spatula or the back of a spoon. Tap the pan gently on the counter to release any air bubbles.

Sprinkle the reserved hazelnuts on top and press them gently into the surface. Dust lightly with cocoa powder using a fine-mesh sieve.

Cover with plastic wrap and refrigerate for at least 4 hours or overnight until completely firm and set.

Lift the fudge out using the parchment overhang and place on a cutting board. Using a sharp knife wiped clean between cuts, slice into 25 small squares or 16 larger squares.

Let the squares sit at room temperature for 5-10 minutes before serving for the best texture and flavor.