Preheat oven to 325°(F). Grease and flour a 9x13 baking pan. Set aside.
In a medium bowl, combine the flour, baking soda, cocoa powder, and salt. Sift together.
In the bowl of your mixer, with the paddle attachment, add butter, oil, and sugar. Cream together on low speed until light yellow and fluffy. Scrap the bowl occasionally.
Add eggs, one at a time, to the creamed butter mixture. Mix well to combine.
In a measuring cup, add buttermilk and vanilla extract. Mix to combine.
Alternate the buttermilk and dry ingredients into the creamed sugar mixture, while mixing on low speed. Mix until well incorporated, scrapping the bowl to insure all the ingredients are well combined.
Stir in the grated zucchini.
Pour cake batter into prepared baking dish. Spread evenly in cake pan. Sprinkle the top of the cake batter with chocolate chips, and then the chopped walnuts.
Bake in preheated oven for 55 minutes or until the center, poked with a toothpick, comes out clean. Remove and cool. Cut to serve. Store covered for up to three days.