Whisk together the flour, baking powder, baking soda, oat flour, salt, nutmeg and cinnamon in a medium bowl.
In a large jug or bowl, mash your bananas with a fork or potato masher. Add in the sugar, melted butter, egg, vanilla and buttermilk and whisk until smooth.
Pour the dry ingredients into the wet and beat until combined and you have no more streaks of flour. Let the batter rest at room temperature for 1 hour.
While the batter is resting, whisk your streusel ingredients together until it is evenly moist.
Preheat the oven to 425°F/220°C. Line a muffin pan with 6 liners.
Fill the liners to the top with the batter. Sprinkle over the streusel topping.
Bake for 5 minutes. Then, without opening the door, reduce the oven temperature to 350°F/175°C and bake for an extra 16-18 minutes.
Allow them to cool completely.
Notes
My recipes have been developed and tested in metric grams. I highly recommend using a weighing scale and measuring out your ingredients the metric way. I have provided a conversion to US cups but please note that I haven't tested the recipe with this method. Just click on 'US Customary' above ingredients. I make sure that all of my ingredients are at room temperature before I start the recipe, so pull out all of the ingredients about an hour or two before you start baking. Salt: If you don't have kosher salt, halve the amount of salt called for in the recipe. 1 teaspoon kosher salt = ½ teaspoon fine salt.Butter: I use European style butter, like Kerry Gold, in my recipes. This will make your finished bake much creamier and richer.Buttermilk: if you don't have buttermilk, then mix ¼ cup of whole milk with ¼ tablespoon of white vinegar or lemon juice and let it sit for 5 minutes. You could also substitute it for sour cream. I do not recommend using just plain milk in this recipe. Oat Flour: if you don't have oat flour then blend rolled oats until you have a fine powder. You could also replace it with an equal amount of flour. I don't recommend using just rolled oats that haven't been blended. Storage: keep in an airtight container at room temperature for 2 days, or in the fridge for five days. Freezing: freeze baked and cooled muffins in a freezer bag. Let them thaw in the fridge or at room temperature.