Heat oven to 325° F. Coat two 9 x 2-inch round baking pans with nonstick cooking spray. Line bottoms with wax paper; coat paper.
In a medium-size bowl, whisk together flour, baking soda and salt.
In a large bowl, beat butter until creamy. Add sugar and beat until fluffy, about 2 minutes. Add vanilla and beat in eggs, 1 at a time, beating well after each addition.
Place unsweetened chocolate and vegetable oil in a small microwave-safe dish and microwave until melted, about 1 minute. Stir until smooth. Beat into butter mixture.
On low speed, beat flour mixture into butter mixture in 3 additions, alternating with buttermilk. Beat on medium 1 minute. Divide batter between pans.
Bake at 325° F for 35 to 38 minutes, until toothpicks inserted in centers come out clean. Cool on wire racks 15 minutes. Turn cakes out onto racks; remove wax paper. Cool completely. Trim cakes with serrated knife if necessary.
Place chopped bittersweet chocolate in a medium-size bowl. Bring cream to a boil and pour over chocolate. Whisk until smooth. Cool to room temperature; cover and chill until good spreading consistency, about 30 minutes.
Place 1 cake layer on a serving plate. Spread Nutella over cake layer; sprinkle 1/2 cup chopped nuts evenly over Nutella. Place remaining cake layer on top. Spread chocolate mixture over top and side of cake.
Garnish with additional hazelnuts, if desired. Let stand at room temperature until set.