Preheat your oven to 350 degrees F. Grease and lightly flour a 12-cup bundt pan.
Beat the granulated sugar, brown sugar, and eggs with a hand mixer or with a stand mixer fitted with the whisk attachment, until the mixture is pale (about 4-5 minutes).
Add the oil, milk, mashed bananas, and vanilla extract to sugar mixture and mix until combined.
Add the flours and baking powder to the mixture and mix on low speed until combined.
Remove 2 ½ cups of batter and transfer it to a separate bowl. In a small mixing bowl, whisk together cocoa powder and hot water until smooth, and then add to the 2 1/2 cups of batter. Mix until the chocolate is fully incorporated into the batter.
Spoon half of the vanilla batter into the prepared bundt pan, then spoon dollops of the chocolate batter on top. Pour the remaining vanilla batter and then use a butter knife to swirl the two batters together.
Bake for 1 hour, or until the top is golden brown and a wooden skewer inserted into the center comes out clean. Let the cake cool in the baking pan for 10 minutes before removing from the pan and transferring it to a cooling rack to cool completely.
Once the cake is no longer hot, dust with powdered sugar (optional), slice, and enjoy!