Preheat your oven to 375 F/190 C/ 170 C fan and place large paper cases in a 12 hole muffin tin.
Sift together 250 grams plain flour, 170 grams caster sugar, 1 tsp bicarbonate of soda, 1 tsp baking powder and 1/2 tsp Shetland sea salt and set aside.
Mash 3 medium bananas in a separate bowl and add 75 grams butter, melted and cooled, and 1 large free-range egg and mix well.
In a third bowl, toss 175 grams blueberries with two spoonfuls of the flour mixture to coat.
Add the banana mixture to the flour mixture and stir with a wooden spoon until just combined. Fold in the blueberries with any flour that might be remaining in the bottom of the blueberry bowl.
Spoon into the prepared muffin cases.
To prepare the streusel topping, combine 75 grams light brown muscovado sugar, 2 tbsp plain flour and 1/4 tsp ground cinnamon together and rub in 1 tbsp butter until the mixture resembles coarse bread crumbs.
Generously sprinkle the streusel topping over each muffin and bake, in the centre of the oven, for 20 - 25 minutes. A skewer inserted into the centre will come out clean.
Transfer to a wire rack to cool completely, or eat warm for breakfast with coffee!