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Preheat oven to 350F. Line an 8x8-inch pan with parchment paper or spray with nonstick spray.
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Melt the butter and semisweet chocolate together in a small saucepan, stirring until smooth. Remove from heat and let cool slightly. Whisk in sugar and eggs. Add flour, cocoa powder, and salt and whisk until just combined. Stir in chopped hazelnuts.
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Pour into the prepared pan. Bake 30 - 35 minutes or until a toothpick inserted into the center comes out clean. Let cool completely before frosting.
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To make the frosting, beat together softened butter, nutella, and milk. Spread over cooled brownies. Sprinkle with chopped hazelnuts.