Preheat oven to 350° Fahrenheit and line a 9×4-inch loaf pan with parchment paper, then lightly coat the inside of the pan with oil.
Place the bananas in a large bowl and mash well with a fork, until mostly smooth.
Add the eggs and vanilla extract (if using) to the bowl the and whisk to combine. Then, add the brown sugar, flour, and sprinkles then mix again until fully combined and no pockets of flour remain.
Pour the batter into your prepared loaf pan and bake for 55-60 minutes, or until a toothpick inserted down the center of the bread comes out clean.
Remove the birthday cake banana bread from the oven and let it cool while you make the glaze.
Add the powdered sugar and milk to a small bowl, then whisk together until smooth. Pour glaze over the cooled banana bread.