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Position rack in center of oven. Preheat oven to 350°F (180°C). Coat the inside of a 9-inch (23 cm) square baking pan with nonstick spray, then line the bottom and two sides with parchment, leaving an overhang on two sides.
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Beat the butter with an electric mixer until smooth and creamy. Beat in brown sugar until lightened in color, soft and creamy, scraping down bowl once or twice. Beat in the vanilla, then beat in the eggs one at a time, allowing each to become incorporated.
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Beat in 1 cup (266 g) Nutella until smooth and creamy. Scrape down the sides and up bottom of bowl as needed. Beat in flour and salt on slow speed until a few floury streaks remain, then use a silicone spatula to fold the flour in completely.
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Scrape the batter into prepared baking pan, smoothing the top; I use a small offset spatula. Drop small spoonfuls of the remaining ¼ cup (66 g) Nutella on top evenly across the batter. Swirl in using a bamboo skewer or butter knife – or the tip of that offset spatula.
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Bake the brownies for about 12 to 20 minutes. I know 12 minutes doesn’t seem long, and they will probably take at least 15 minutes BUT these bake quickly, and they can dry out easily. Many batches at 20 minutes were over-done. A toothpick inserted in the center should have moist crumbs clinging and not come out clean.
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Cool pan on rack for a few minutes then place in fridge to quickly chill them down, which halts residual heat. (When I made these in the winter, I put them on my outdoor porch!) Cool completely in the pan then use the parchment to lift them out, remove parchment and cut into a 6 by 6 grid for 36 brownies. Store in an airtight container at room temperature for up to 3 days. They can also be frozen for up to 1 month. Double wrap in plastic wrap, then place in a zip-top bag.