Heat the oven to 350°F (180°C), conventional setting. Line a standard 12-cup muffin pan with paper liners. Measure the pumpkin purée and mashed banana before beginning.

Whisk the flour, baking powder, baking soda, cinnamon, ginger, nutmeg, and salt in a large bowl. Distribute the leaveners well so the muffins rise evenly.

In a second bowl, whisk the pumpkin, banana, eggs, oil, brown sugar, and vanilla until combined. Break up large lumps of brown sugar. Do not taste raw batter containing flour and eggs.

Add the wet mixture to the dry ingredients and fold gently with a spatula just until there are no dry flour pockets. Stop mixing at that point; a few small lumps are fine.

Divide the batter evenly among the twelve cups, filling them about three-quarters full. Bake for about 18–22 minutes, until the tops spring back lightly and a toothpick inserted in the center comes out without wet batter. If the center is wet, continue baking and check again.

Cool the muffins in the pan for 5 minutes, then transfer them to a rack for at least another 5 minutes. Let them cool completely before sealing a storage container so condensation does not make the tops sticky.