Preheat the oven to 220 °C.
Sift the baking soda, sugar, flour, baking powder and salt into a suitable bowl, add the lemon zest and brown sugar.
Mix the banana with the eggs, buttermilk, vanilla and melted butter. Add the mixture to the dry ingredients and fold in a few quick strokes. The batter will be lumpy, but you still can’t over mix or the muffins will be hard!
Carefully fold in 3/4 of the blueberries, stirring repeatedly very briefly (5 to 6 times). If the batter is completely smooth, you have stirred too much.
Fill the molds 3/4 full, decorate with the remaining blueberries and sprinkle with sugar and cinnamon. Bake on middle rack at 220 °C for 25 min.