Preheat the oven to 425ºF (220ºC). Generously grease a 12-cup muffin tin with butter. Line the tin with paper muffin wrappers and set aside.
Place the flour, baking soda, and salt in a medium-large bowl. Whisk until well combined. Set aside.
Add the butter and both sugars to a mixing bowl. Use a hand or stand mixer to cream the butter and sugars together on medium-high speed until the mixture is light and fluffy, about 1 to 2 minutes.
Add the eggs to the bowl, one at a time, mixing well and scraping down the sides of the bowl between each addition.
Add the bananas to the bowl and mash them smooth with a fork. Add the pumpkin purée and vanilla extract to the bowl. Mix well.
Add the flour mixture to the bowl. Switch to a spoon or spatula and gently fold until the flour is just mixed in.
Scoop or spoon the batter evenly between the 12 prepared muffin cups.
To make the streusel, stir the brown sugar and flour together in a small bowl. Use a pastry cutter or two knives to quickly cut the cold butter into the dry mixture until you see pea-sized crumbs, keeping the streusel cool and crumbly.
Sprinkle all of the streusel evenly over the 12 pumpkin banana muffins. Place the tin in the preheated oven and bake the muffins for 15 minutes. Reduce the heat to 350ºF (180ºC) and continue to bake 5 to 7 minutes more. Transfer the tin to a wire rack to cool for 10 minutes before turning the muffins out onto the rack to cool completely.
Notes
For detailed ingredient notes, substitution tips, variations, and step-by-step photos, scroll through the full recipe post.
Use a digital kitchen scale for precise measurements and an oven thermometer to confirm your oven is heating correctly.
Let your bananas ripen until the skins are very dark, almost black, for the best flavour.
Mix the streusel just until crumbly, leaving some larger clumps for crunch.
Storage
Store any leftovers in an airtight container at room temperature for up to two days.
For longer storage, place the muffins in a sealed container in the freezer for up to two months.