Preheat oven to 325 degrees F (165 degrees C). Grease a 9x13-inch cake pan. Stir flour, cocoa powder, salt, and baking soda together in a bowl. Mix sugar, butter, vegetable oil, and eggs together in a large bowl. Add chile powder and vanilla extract. Pour flour mixture into sugar mixture; add buttermilk and stir to combine. Mix zucchini into batter. Pour batter into the prepared cake pan; sprinkle chocolate chips and walnuts on top. Bake in the preheated oven until a toothpick inserted in the center comes out clean, about 55 minutes.