Preheat the oven to 425°F/220°C. Line a muffin pan with 12 liners
Mix together the streusel ingredients and set aside.
Whisk together the flour, oat flour, baking powder, baking soda, salt, cinnamon and nutmeg in a medium bowl or jug.
In the bowl of a stand mixer with the paddle attachment, mash the bananas until it’s the consistency of applesauce
Add in the sugar, eggs, butter, vanilla and milk then beat on medium speed. It is going to look a bit chunky!
Pour the dry ingredients into the wet and beat until combined, you will still have a few lumps in the batter.
Keep a handful of blueberries to the side. Fold the rest of them into the batter
Fill the liners to the top with the batter. Add a few blueberries to the top of each muffin and sprinkle over the streusel
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.
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 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.Storage: keep in an airtight container at room temperature for 2 days, or in the fridge for four days. Freezing: freeze baked and cooled muffins in a freezer bag for up to 2 months. Let them thaw in the fridge or at room temperature.