Preheat oven to 350 degrees F. Lightly coat an 8-inch springform pan with cooking spray. In a large bowl, whisk together cocoa powder, flour, sugar, baking soda, baking powder and salt. Whisk in egg yolk, buttermilk, 1/4 cup warm water, oil and vanilla. Pour batter into pan and bake until cake pulls away from sides and a toothpick inserted in center comes out clean, about 15 minutes. Let cool completely in pan on a wire rack, 1 1/2 hours.