Heat oven to 325 and grease two 9 inch round pans. In a medium bowl, whisk together flour, baking soda and salt. In a large bowl, beat butter until creamy. Add sugar and beat until fluffy. Add vanilla and beat in eggs, one at a time.
Melt unsweetened chocolate and vegetable oil. Add to butter mixture. On a low speed, beat flour mixture into butter mixture in three additions, alternating with buttermilk. Divide between pans and bake for 35 to 38 minutes.
Put bittersweet chocolate in a bowl. Bring cream to a boil and pour over chocolate. Whisk until smooth. Cool to room temperature; cover and chill until a good spreading consistency.
Spread Nutella over one cake, sprinkle with 1/2 cup chopped nut. Top with second cake. Frost cake. Garnish with extra nuts.
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