Preheat oven to 350°. Butter and flour a 9-inch square baking pan. Tap out excess flour. Set pan aside.
Toast and skin hazelnuts. Chop hazelnuts in a food processor until coarsely ground. Hazelnut pieces should be about 1/8-inch.
Coarsely chop chocolates and place in the top of a double boiler. Melt with butter and Nutella over simmering water. Stir occasionally until smooth. Remove top bowl from heat for mixture to cool.
Sift together flour, baking powder, and salt. Set aside.
Whisk sugar into chocolate mixture, mixing well. Whisk in eggs until mixture is glossy and smooth. Stir in flour mixture and hazelnuts, stirring just until combined.
Pour batter into prepared pan. Bake for about 35 minutes, or until a tester comes out with moist crumbs. Cool brownies completely in pan on a wire rack.