Preheat oven to 400 F (200 C) for conventional oven, 380 F (190 C) for convection oven. Line your muffin pan with liners
Prepare the streusel:
Use a fork to combine all the ingredients for the streusel. There will be some uneven sizes of crumbles and that's okay. Keep it chill in the fridge while you prepare the batter
Prepare the batter:
Whisk to combine all the dry ingredients in a mixing bowl.
In another bowl, mash the bananas with a fork. They don't have to be very smooth. I like to have small chunks of banana in the muffins. Add the rest of the wet ingredients and whisk to combine
Make a well in the center of the dry ingredients and pour in the wet ingredients. Use a rubber spatula to fold the dry into the wet while rotating the mixing bowl. While most of the dry bits are almost combined, add the chopped nuts
Continue to fold until you no longer see dry bits of flour. Stop mixing. You don't want to overmix the batter
Scoop the batter into the prepared muffin pan. Fill it up until almost to the top, leaving a bit of space for the streusel topping.
Sprinkle the streusel on each one of them
Bake:
Put the muffin pan on the center rack in the oven. Bake for 18-20 minutes or until a cake tester inserted into the center of the muffin comes out clean with just a bit of crumbs is okay as long as it's not wet. Do not overbake muffins. They will be dry
Cooling:
They are still very fragile and soft when they are first out of the oven. Let the muffins cool in the pan for 5 minutes. Then carefully transfer them to a cooling rack to cool down further
Serving:
The muffins can be served warm or at room temperature
Storing:
Store the muffins in an air-tight container for 2-3 days. They get even better the next day. They are more moist and flavorful as the flavor has melded.
For longer storage, keep them wrapped individually with a cling wrap and store them inside a freezer bag, push all the air out and seal. They can be kept frozen for one month. Thaw at room temperature before serving