Preheat the oven to 325 degrees F. Coat a 9" x 13" baking pan with the oil spray. Cut a 13" x 13" piece of parchment paper to fit the bottom and up 2 of the 4 sides of the pan. (The parchment paper will later be used as handles to lift the cake out of the pan.)
Place the flour, cocoa, baking soda and salt in a bowl and whisk to combine. Mix in the chocolate chips and pecans. In another bowl, beat together the butter and sugar until well combined. Mix in the eggs, vanilla and buttermilk. Mix in the dry ingredients until well combined. Stir in zucchini, and then pour and spread the batter into the pan. Bake for 45 to 50 minutes, or until a toothpick inserted into the centre of the cake pulls out clean. Cool cake on a baking rack to room temperature. When ready to serve, grab onto the parchment paper and lift the cake out of the pan and set on a cutting board. Dust the top of the cake with icing sugar. Cut the cake into squares and serve.