Heat oven to 350F. Spray bundt pan with nonstick cooking spray.
In a mixing bowl whisk together the flours, baking powder, baking soda, and salt.
In another mixing bowl (I used my Kitchenaid), combine the oil, sugar, and bananas. Beat until smooth. Mix in the vanilla, eggs, and yogurt. Blend until smooth.
Fold wet ingredients in dry ingredients, combining just until blended.
Spoon half the batter into the greased bundt pan.
Add the cocoa powder to the remaining batter in the mixing bowl and stir until just combined. Spoon the chocolate batter in dollops around the bundt pan, leaving some lighter colored spaces in between.
Use a knife to swirl the different colored batters together, taking care not to mix them too much.
Bake the for 35-40 minutes or until tester comes out with just a few crumbs attached. Cool the cake in the pan on a wire rack for 15 minutes. Carefully, run a rubber spatula around the edges of pan to loosen cake. Invert the pan onto a cooling rack and remove pan. Cool completely. Transfer to serving dish. Sift powdered sugar over top.
To freeze: Once the cake has cooled, wrap carefully in plastic wrap and place in a large ziptop bag. Remove as much air from the bag as possible and freeze. To serve, thaw completely. Sift over powdered sugar before serving.