Step 1: In a small bowl, prepare the streusel topping by combining 1/4 cup all-purpose flour, 1/4 cup packed light brown sugar, 1/2 teaspoon ground cinnamon, and a pinch of salt. Whisk until well combined.
Step 2: Add 2 tablespoons of cold, unsalted butter, cut into small cubes, to the streusel dry ingredients. Use your fingertips or a pastry blender to cut the butter into the mixture until coarse crumbs form. Set aside.
Step 3: Preheat your oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease it well.
Step 4: In a medium bowl, mash the 3 very ripe medium bananas with a fork until mostly smooth but with a few small lumps remaining.
Step 5: To the mashed bananas, add 1/2 cup granulated sugar, 1/4 cup packed light brown sugar, 1 large egg (lightly beaten), 1/4 cup unsalted butter (melted and slightly cooled), 1/4 cup milk, and 1 teaspoon vanilla extract. Whisk until just combined.
Step 6: In a separate large bowl, whisk together 1 1/2 cups all-purpose flour, 1 teaspoon baking soda, 1/2 teaspoon ground cinnamon, 1/4 teaspoon ground nutmeg, and 1/4 teaspoon salt. Ensure there are no lumps.
Step 7: Pour the wet ingredients mixture into the dry ingredients. Using a rubber spatula, gently fold until just combined. Be careful not to overmix; a few streaks of flour are acceptable.
Step 8: Divide the muffin batter evenly among the prepared muffin cups, filling each about two-thirds full.
Step 9: Evenly sprinkle the prepared streusel topping over the batter in each muffin cup.
Step 10: Bake for 18-22 minutes, or until a wooden skewer or toothpick inserted into the center of a muffin comes out clean.
Step 11: Let the muffins cool in the muffin tin for 5 minutes before transferring them to a wire rack to cool completely. Serve warm or at room temperature.