Chocolate Walnut Layer Cake - Diamond Nuts

Ingredients

• 1 2/3 cups all purpose flour
• 3/4 cup unsweetened natural cocoa powder
• 2 teaspoons baking powder
• 1 teaspoon baking soda
• 1 teaspoon salt
• 1/2 cup toasted, finely chopped walnuts
• 2 cups sugar
• 12 tablespoons unsalted butter, room temperature
• 2 teaspoons vanilla extract
• 2 eggs
• 2 cups buttermilk
• 10 ounces bittersweet chocolate
• 4 tablespoons unsalted butter, room temperature
• 1/2 cup powdered sugar
• 1 teaspoon vanilla extract
• 1/4 teaspoon salt
• 3/4 cup sour cream
• 1 1/2 cups toasted walnuts, coarsely chopped