Preheat oven to 350°F and generously grease a bundt pan with cooking spray. Set aside.
In a liquid measuring cup (or large bowl), mash the bananas. A few lumps are ok.
In a large bowl, mix together the bananas, eggs, yogurt, vanilla extract, and sugar. You can do this with a stand mixer or hand mixer on medium speed, or by hand.
Slowly add in the dry ingredients (baking powder, baking soda, salt, ground flaxseed, all-purpose flour, whole wheat flour) and stir to combine. Once combined, stir in the chocolate chips until well dispersed throughout the batter.
Pour the batter into the prepared bundt pan. Bake 45-55 minutes (check at the 45 minute mark) until the cake is done. You can tell the cake is done when the edges start to pull away from the pan and the top feels firm. You can also check it with a toothpick, it is done with the toothpick comes out clean.
Allow the cake to cool at least 10 minutes, and then invert the cake onto a wire rack to cool completely.