Heat oven to 375°F. Use muffin liners to line 18 cups of two 12-cup muffin pans. Spray the pan with cooking spray.
Whisk pumpkin, banana, buttermilk, oil, brown sugar, eggs, and vanilla in a large bowl.
In another bowl, whisk flour, sugar, cinnamon, baking powder, baking soda, and salt.
Make a well. Add wet ingredients to dry. Stir gently until a few flour streaks remain. Do not overmix.
Portion batter to fill 18 muffin cups nearly full. Bake 20 to 25 minutes until the internal temperature reaches 190ºF. If you don't have a cooking thermometer, they'll be golden and bouncy when gently pressed when done.
Cool in pan 5 minutes, then transfer to a rack.
Notes
Spray or grease the muffin tin even if you're lining with paper liners. This ensures that any batter overflow doesn't stick.I love to make big, beautiful bakery-style muffins so I fill the cups nearly to the brim (leaving about 1/4" headspace) but if you prefer to make smaller muffins, fill all 24 muffin cups 3/4 full. They'll be done in 18-22 mins.