• Preheat oven to 350 degrees. Grease a 9-inch springform pan and line with parchment paper round; set aside.
• In a medium bowl, whisk together cocoa powder and boiling water until the mixture is smooth. Set aside to cool.
• In a separate medium bowl, whisk together the all purpose flour, baking powder, baking soda, and salt. Mix until well combined and set aside.
• In a large bowl, combine the granulated sugar, oil, and egg, and mix together using an electric mixer until mixture is light and fluffy; about 1 minute. Add in the sour cream and vanilla, and mix until combined. Pour in cooled cocoa powder/water mixture and mix until well combined.
• Add the dry ingredients into the wet and mix until the batter just comes together, using a rubber spatula to scrape down the sides of the bowl. Fold in the ½ cup of chopped walnuts and mix until combined.
• Pour the batter into the prepared springform pan. Place on the center rack of the preheated oven and bake for about 35-40 minutes, or until a toothpick inserted in the middle of the cake comes out clean. Be careful not to over bake the cake. Remove from oven and let cool on wire rack for 10 minutes. Then, remove the collar from the pan and let cool completely.

Chocolate Ganache

• Place the chocolate chips in a bowl. Heat the heavy cream, either in the microwave or on the stove, just until it reaches a low simmer. Remove from heat and pour over the chocolate chips. Cover the bowl with a plate or plastic wrap and let the mixture sit for 5 minutes. Whisk together the cream and chocolate until you reach a smooth pourable consistency.
• Pour the chocolate ganache on top of the cooled chocolate walnut cake, allowing some of the frosting to drip down the sides. Garnish with remaining ½ cup of chopped walnuts. Let the cake sit for a few minutes to allow the ganache to set before slicing and serving.