Pour both sugars into the cooled butter and whisk together.
½ cup light brown sugar, ½ cup granulated sugar
Whisk in the eggs, mashed bananas, sour cream, and vanilla extract.
1 ½ cups mashed bananas, 2 large eggs, ⅓ cup sour cream, 1 teaspoon vanilla extract
Pour the dry ingredients in and use a rubber spatula to fold together until just combined.
Let the muffin batter rest for 30-60 minutes.
While the muffin batter rests, make your filling and streusel.
Whisk the cinnamon sugar filling ingredients together in a small bowl and set aside.
¼ cup light brown sugar, 1 teaspoon ground cinnamon
In a jug or medium bowl, mix the melted butter with the remaining streusel ingredients. It should become thick and clumpy. Place in the fridge while you wait for the muffin batter to finish resting.
¼ cup unsalted butter, ¾ cup all purpose flour, ¼ cup light brown sugar, 1 teaspoon ground cinnamon, ¼ teaspoon ground cardamom, ¼ teaspoon kosher salt
Preheat the oven to 425°Fahrenheit/218°Celsius and adjust the rack to be in the middle of the oven. Place 12 liners in 2 muffin pans, leaving every other muffin cavity empty.
Remove the plastic wrap from the muffin batter bowl. Using a medium cookie scoop or a spoon, gently scoop out 1 ½ tablespoons of muffin batter and place in the prepared muffin pan. Sprinkle over ½ teaspoon of cinnamon sugar filling.
Scoop out another 1 ½ tablespoons of muffin batter and place on top of the cinnamon sugar filling. At this point you should have filled the liners up to the top.
Sprinkle over the chilled streusel topping. You may need to use a fork or your hands to break it up. Sprinkle over a bit of extra cinnamon sugar filling.
Bake in the preheated oven for 5 minutes, then lower the temperature (without opening the door) to 350/175 and bake for an extra 14-16 minutes. Let the muffins cool in the pan for 10 minutes before removing to a wire cooling rack to cool completely.
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Notes
My recipes have been developed and tested in metric grams. I highly recommend using a weighing scale and measuring out your ingredients the metric way. Cardamom: If you can't find ground cardamom you can swap it out for ground ginger or leave it out completely. Salt: If you don’t have kosher salt, halve the amount of salt called for in the recipe. 1 teaspoon kosher salt = ½ teaspoon fine salt. Please note I am using Diamond Crystal Kosher Salt and not Morton Salt, which is a coarse kosher salt. Coarse kosher salt is much too salty for this recipe. Baking 6 at a time: by only filling every other muffin cavity, you'll get much taller muffins. I don't recommend baking them 12 at a time. Storage: keep in an airtight container at room temperature for 2 days, or in the fridge for five days. Freezing: freeze baked and cooled muffins in a freezer bag. Let them thaw in the fridge or at room temperature.