Preheat the oven to 350°F/180°C and grease a 10 cup capacity bundt pan well and set aside.
Slice the bananas and place them in a medium-sized microwave-safe bowl with the butter and condensed milk. Microwave on high for 2 minutes (the butter may not be completely melted) Mash with a fork or potato masher until the banana is fully mashed then mix in the vanilla, buttermilk, and eggs (make sure the mixture isn't too hot or it will cook the eggs). Set aside.
In a large mixing bowl mix the dry ingredients together until well combined, make a well in the middle, and pour in the banana mixture. Mix until combined.
Add the batter to the prepared bundt pan and bake for 35-45 minutes or until a cake tester comes out clean. Place the pan on a wire rack until it is cool enough to handle, then carefully unmold the cake and place it back on the rack to cool completely.