Preheat the oven to 350°F (175°C). Line an 8×8-inch metal baking pan with foil or parchment paper, 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.