Preheat the oven to 350-F degrees and lightly oil two 9-inch round cake pans, or give them a light coating of butter. Dust the pans with flour and tap out any excess.
In a small mixing bowl, whisk together the boiling water and the cocoa powder until smooth. Set aside to let cool.
In a medium mixing bowl, sift together the cake flour, baking soda, baking powder, and salt. Set aside.
In a large mixing bowl, blend together the granulated sugar, canola oil, butter, and vanilla extract. Beat with an electric mixture at medium speed until blended. Beat in the egg whites one at a time, then add the yogurt and increase the speed to high. Blend until the mixture is light and fluffy, about 2 or 3 minutes.
Reduce the mixer speed to low and add in half the dry ingredients. Increase the speed to medium and continue to blend. Reduce the speed again, and add half of the reserved cocoa liquid. Repeat the process with the remaining dry ingredients and cocoa liquid, making sure to scrape the sides of the bowl occasionally.
Divide the batter evenly among the two prepared cake pans. Lightly tap the filled cake pans lightly on the counter to remove any large air bubbles. Bake for 25 to 30 minutes, or until a cake tester is cleanly removed from the center of each cake.
Let the cakes cool in the pan on a wire rack for about 10 minutes. Loosen the edges slightly with a sharp knife, then invert the cakes onto an oiled wire rack to cool completely. The cakes will keep for up to 2 days in an airtight container, or up to 2 months when frozen.