Preheat the oven to 350 degrees F. Line a 12-count muffin pan with cups/liners or spray with non-stick spray, then set it aside.
Mix the wet base
In a large bowl, mash the bananas with a fork until mostly smooth.
Add the sugar and softened butter to the bananas, then beat with an electric hand mixer (or mix by hand) until combined and creamy.
Add the eggs and vanilla, then mix again until smooth and glossy.
Combine dry ingredients, then fill the tin
In a medium bowl, add the flour, baking powder, baking soda, and salt, then whisk to sift and evenly distribute the leaveners.
Gradually add the dry mixture to the wet mixture, stirring just until everything comes together, stopping as soon as you no longer see dry flour to avoid tunnels.
Using an ice cream scoop, divide the batter evenly among the 12 muffin cups, filling each liner about 3/4 full.
Add streusel, bake, and cool
In a separate bowl, mix together all of the streusel topping ingredients, then sprinkle it over each batter-filled muffin cup, using a generous hand for that cinnamon streusel scent.
Bake for 20 minutes, until the tops look set and lightly golden.
Carefully remove from the oven and let the muffins stand at least 10 minutes before removing from the pan. Serve and enjoy.
Notes
For consistent results, aim for about 1.5 cups of mashed bananas (approximately 375 g). Stir the batter just enough to remove all traces of flour to prevent tunneling. Let it rest in the pan for 10 minutes before unmolding.Nutrition information is estimated based on common ingredients and serving sizes and may vary.