Spray the bundt cake form with a baking spray or see my note below for a homemade bundt cake paste. Set aside.
Preheat the oven to 350 degrees F.
In a large mixing bowl, whisk together the dry ingredients (flour, both sugars, baking powder, baking soda and salt). Set aside.
In another mixing bowl, mash the bananas. Add buttermilk, oil, vanilla and eggs. Beat with an electric mixer until smooth.
Add the wet ingredients to the dry ingredients and mix well. Beat for 2 minutes.
Pour the cake batter into the prepared pan.
Bake for 45 to 50 minutes, or until the toothpick inserted in few places comes out clean.
Cool the cake in the pan for 2 minutes.
Place a plate or a cooling rack on top of the bundt pan. Invert and let the cake sit in the pan on the cooling rack. Tap gently on the sides. If the cake doesn't slip right out, let it sit for 10 minutes. Then try again.
!to make the glaze:
In a medium saucepan, combine butter and milk. Sift the cocoa powder right over the saucepan. Cook the mixture over medium heat, whisking constantly. The butter should melt and the mixture thicken up. Remove the saucepan from the heat.Cool for 10 minutes.
Sift the powdered sugar over the saucepan. Whisk it in. Add vanilla and whisk in well.
Place the cake on a cooling rack set over a rimmed baking pan.
Pour the glaze over the cooled cake.
Let the cake sit for at least 30 minutes so the glaze will set.