Preheat the oven to 425°F (218°C). Line two 12-well muffin pans with about 18-20 paper liners and set aside.
In a medium bowl, stir together the flour, baking powder, and salt and set aside.
In a large mixing bowl, using a whisk, mix together the mashed bananas, sugar, and oil until well combined, about 1-2 minutes. Then add the eggs, cinnamon, and vanilla and stir to combine, about 1 minute. Add the flour mixture and whisk until thoroughly combined, about 1 more minute. Then fold in the walnuts and banana chunks just to incorporate and set aside.
In a small bowl, use your fingers to combine the flour, brown sugar, walnuts, cinnamon, and butter until the mixture is crumbly. Do not overwork. Otherwise, it will become a paste rather than crumbs.
Spoon the batter into the prepared paper liners until almost full and top with the streusel. You will end up with about 18 muffins.
Bake at 425°F (218°C) for 5 minutes. Then, leaving the oven door closed, lower the heat to 350°F (177°C) and bake for another 11-13 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove from the oven and let cool in the pan for 15 minutes. Then transfer to a wire rack and let cool completely. Store in an airtight container at room temperature for up to 3 days or freeze for up to 2 months.