Preheat oven to 425°F. Line a jumbo 6-cup muffin tin with liners or grease generously.
In a large bowl, mash the ripe bananas until mostly smooth.
Add maple syrup, oil, eggs, vanilla, and sour cream to the bananas. Whisk until well combined.
Add flour, baking soda, baking powder, cinnamon, and salt. Fold with a spatula until just combined — do not overmix.
Fold in most of the walnuts, reserving a small handful for the tops.
Divide batter evenly between the 6 muffin cups, filling nearly to the top. Press reserved walnuts onto each muffin.
Bake at 425°F for 7 minutes. Without opening the oven, reduce temperature to 350°F and bake an additional 15 minutes, until golden and a toothpick inserted in the center comes out clean.
Cool in the pan for 15 minutes before removing and serving.