Preheat the oven to 350°F. Line an 8x8-inch *metal* baking pan with foil or parchment, leaving an overhang, and spray with nonstick cooking spray.
In a large microwave-safe bowl, combine the chocolate chips, butter, and unsweetened chocolate. Heat in the microwave for 1 minute then stir. Continue heating in 30-second bursts, stirring between each burst, until the mixture is melted and smooth but not scorched. Remove from the microwave and let cool slightly. Stir in the Nutella.
In a large bowl, use an electric mixer to beat the eggs and sugar on medium-high speed until light and thick, about 3 minutes. Beat in the vanilla extract. Stir in the cooled chocolate mixture with a rubber spatula. Add the flour, cocoa, and salt, folding gently until combined.
Pour the batter into the prepared baking pan. Bake until a toothpick or tester comes out with moist crumbs still attached, 35 to 40 minutes. Do not overbake, you want to err on the side of underbaking. Let cool to room temperature. Serve at room temperature, warmed, or chilled.
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Notes
Nutella Brownies taste best the day they are baked but can be stored in an airtight container for up to 3 days at room temperature or up to 5 days in the fridge.Brownies are even more fudgy when they’re chilled!The Nutella flavor is slightly muted with baking. To punch it up, add a tablespoon of Frangelico to the batter or 1 cup of toasted chopped hazelnuts, if you want a bit of crunch.