Preheat the oven to 360 F / 180 C and lightly grease a muffin pan with oil.
Mash the bananas in a bowl. Add the sugar, oil and vanilla and mix well to combine.
In a separate large bowl, combine the flour, baking soda and salt.
In a third bowl, make the crumb by combining all the ingredients and use a fork to cut in the butter until the mixture is crumbly.
When the oven is hot, pour the banana mixture into the flour mixture and mix until just combined. Do not over mix. Fill each cup of the muffin tin. They say you should only fill them ¾ full, I fill them to the top and don’t have a problem with flat muffins, however I get 10 rather than 12 muffins.
Sprinkle the crumb over each muffin and bake for 25 minutes or until a toothpick comes out clean.
Meanwhile, if you are making the glaze (optional) put the icing sugar in a small bowl and add one teaspoon of milk, mix well. If it is too thick, add another teaspoon of milk. Make it thin enough to run off the spoon but not so thin it runs off the muffins too.
Let the muffins cool completely then drizzle over the glaze.