Preheat oven to 350°F. In 9-inch round baking pan, whisk together whole-wheat pastry flour, all-purpose flour, sugar, cocoa powder, baking soda and salt.
In small bowl or measuring cup, combine water and vinegar.
Make well in center of flour mixture in pan and pour canola oil and vanilla extract into well. Pour water-vinegar mixture over top of flour and then stir well to blend all ingredients.
Bake until set and toothpick inserted into center comes out clean, about 30 to 35 minutes. Place cake on rack to cool in pan. Once cool, sprinkle with confectioners’ sugar.