Preheat your oven to 350°F. Line an 8×8-inch metal baking pan with foil or parchment, leaving an overhang, and coat it with nonstick cooking spray.
In a large microwave-safe bowl, combine the chocolate chips, butter, and unsweetened chocolate.
Microwave for 1 minute, then stir.
Continue heating in 30-second intervals, stirring each time, until the mixture is fully melted and smooth.
Allow it to cool slightly, then mix in the Nutella.
In another large bowl, use an electric mixer to beat the eggs and sugar on medium-high speed until the mixture is light and thick, about 3 minutes.
Add the vanilla extract and mix well.
Gently fold in the cooled chocolate mixture with a rubber spatula.
Then, incorporate the flour, cocoa, and salt until just combined.
Pour the batter into the prepared pan and bake for 35 to 40 minutes, or until a toothpick inserted into the center comes out with moist crumbs.
Avoid overbaking to keep the brownies fudgy. Allow to cool to room temperature before serving, either at room temperature, warmed, or chilled.