Baked uncovered in the oven with a water bath to keep it moist, this dessert's creamy texture is reminiscent of a rich custard. Beaten egg whites make it lighter and airier than other steamed puddings.
Heat oven to 325 degrees. Butter a 5-cup pudding bowl, and set aside.
Combine 5 tablespoons sugar with salt, flour, and cocoa in a large bowl. Set aside.
Combine chocolate and butter in a heat-proof bowl, and melt over simmering water.
In a small saucepan, bring milk just to a simmer. Immediately add to chocolate mixture. Beat egg yolks. Stir a little chocolate mixture into yolks; add yolks to mixture. Add vanilla; stir until well combined. Stir into flour mixture.
Beat whites with remaining 3 tablespoons sugar until stiff, and fold into batter. Pour into pudding bowl, and place on a rack in a deep roasting pan. Pour boiling water into pan halfway up the sides of the bowl. Bake until pudding is set, 40 to 45 minutes. Add boiling water to pan as necessary to maintain water level.
Let cool slightly in the bowl, and invert onto a plate. Garnish with hazelnuts.
Serve warm from the bowl or allow to cool slightly and invert onto a plate.