Preheat your oven to 375 degrees F (about 190 degrees C). Make sure it’s fully preheated before you pop your muffins in for even baking.
Lightly grease 10 cups in your muffin pan or line them with muffin liners. This will prevent the muffins from sticking.
In a large bowl, combine the dry ingredients: 1 1/2 cups flour, baking soda, baking powder, and salt. Mix thoroughly to ensure that all the leavening agents are evenly distributed.
In another bowl, beat together the bananas, white sugar, egg, vanilla, cinnamon, nutmeg, and melted butter. You want to mash the bananas well for a smooth mixture.
Stir the wet ingredients into the flour mixture just until moistened. Be careful not to overmix; it’s okay if there are a few lumps!
Spoon the batter into the prepared muffin cups, filling each about three-quarters full. This allows room for the muffins to rise without overflowing.
To prepare the crumb topping, take a small bowl and mix together the brown sugar, 1/4 cup flour, and 1/2 teaspoon cinnamon. Cut in the 2 tablespoons butter until the mixture resembles wet sand. You can use your hands for this, which helps combine everything better.
Sprinkle this streusel topping generously over the muffins. It adds a lovely crunch and sweetness to each bite.
Bake in the preheated oven for 15 to 20 minutes, or until a toothpick inserted into the center of a muffin comes out clean. Keep an eye on them; every oven is different!
Once done, remove the muffins from the oven and let them cool for a few minutes in the pan before transferring them to a wire rack. This allows them to set without getting soggy.
Store the cooled muffins in an airtight container to keep them fresh. You can also freeze them for later enjoyment!
