Preheat your oven to 350°F (175°C). Grease and flour your bundt pan thoroughly to prevent sticking.
In a large mixing bowl, whisk together the flour, granulated sugar, brown sugar, baking powder, baking soda, and salt until well combined.
In a separate bowl, mix the mashed bananas, buttermilk, vegetable oil, vanilla extract, and eggs until smooth.
Pour the wet ingredients into the dry ingredients and mix until just combined. Be careful not to overmix; a few lumps are okay!
Gently pour the batter into the prepared bundt pan, smoothing the top with a spatula.
Bake in the preheated oven for 55-60 minutes, or until a toothpick inserted into the center comes out clean.
Once baked, remove the cake from the oven and allow it to cool in the pan for about 10-15 minutes. Invert it onto a cooling rack to cool completely.
In a small saucepan or microwave, melt the unsalted butter. Whisk in the cocoa powder, followed by the milk and vanilla extract. Gradually add the powdered sugar until the glaze is smooth and pourable.
Once the cake is completely cool, pour the chocolate glaze over the top, allowing it to drip down the sides.