In a bowl, whisk all the wet ingredients, salt and sugar until well combined. (Add flaxmeal to the wet for AP version)
In another bowl, whisk the flours, baking powder and spices and add to the wet ingredients.
Whisk until just about combined into a flowy batter. Use a few teaspoons more water or flour if needed, since the water absorption depends on the flours.
Transfer half of the cake mix to another bowl and add cocoa powder to it. Mix till combined.
The cocoa powder will thicken the batter a little, Add 2-3 teaspoons water to the cocoa batter to get it to the same consistency as the white batter.
Take 2 ladles or small bowls of the same size and pour a ladle of the white cake mix onto parchment lined or well greased cake pan.
Then pour a ladle of cocoa mix on top of the white mix.
Continue till all mix gets used up.
Bake in preheated 360 degrees F for 35-40 minutes until toothpick from the center comes out clean.
Serve as is, or topped with melted chocolate.
Cool for a bit before slicing for less crumbly slices.
Notes
Nutritional values for one serving of half whole wheat version