Line an 8x8-inch baking pan with parchment paper, leaving overhang for easy removal.
In a medium saucepan over low heat, combine chocolate chips, sweetened condensed milk, and butter.
Stir continuously until fully melted and smooth.
Remove from heat and stir in hazelnut extract or emulsion, vanilla extract, and salt.
Fold in chopped hazelnuts, reserving a small handful for topping if desired.
Pour fudge into prepared pan and smooth the top.
Sprinkle remaining hazelnuts over the surface and gently press them in.
Refrigerate for at least 2 hours or until fully set.
Lift fudge out using parchment, slice into squares, and serve.