Preheat your oven to 425°F.
Add the flour, baking soda, and salt to a medium bowl and whisk well. Set aside.
3 cups all-purpose flour, 1½ tsp baking soda, ⅓ tsp salt
Mash the bananas in a bowl or large glass measuring cup (makes it easier to measure as you go!). You should end up with approx. 1½ cups of mashed bananas. Set aside.
1½ cups mashed bananas
In the bowl of your stand mixer (or in a large mixing bowl using a hand mixer), cream together the softened salted butter and white granulated sugar on low speed for about 3-4 minutes, or until smooth.
¾ cup salted butter, 1½ cups white granulated sugar
Add the eggs one at a time, mixing on low speed. Scrape down your bowl.
3 eggs
Add the vanilla extract and sour cream. Mix until well combined.
1½ cups sour cream, plus 2 teaspoons, 1½ tsp vanilla extract
Add the mashed bananas. Mix on low until combined. Then, scrape down your bowl well.
Spoon in the dry ingredients about ½-1 cup at a time. Mix on low until just combined. Then, remove the bowl from the stand mixer and use a rubber spatula to scrape down the bowl and mix in any pockets of dry ingredients that the mixer might have mixed.
Make your quick streusel. In a small bowl, whisk together the flour and brown sugar. Pour in the melted butter and vanilla extract. Use a fork to roughly mix the streusel, almost chopping and slicing it as you mix to create lumps of various sizes. Do not overmix as this will cause the streusel to turn to a paste.
1 cup all-purpose flour, ½ cup light brown sugar, 5 tbsp salted butter, ½ tsp vanilla extract
Add liners to a regular muffin tin and spray the insides with baking spray. Spoon ¼ cup muffin batter into each cup, then top with 1-2 generous teaspoons of streusel (or to taste), piling it slightly in the center.
Bake the muffins at 425°F for 5 minutes. Without opening the oven, turn the heat down to 350°F. Bake for an additional 12-14 minutes, or until a toothpick inserted into the center of the muffins comes out clean or with a few moist crumbs stuck to it. Do not overbake!
Allow the muffins to cool in the tin for 3-4 minutes before carefully (use oven mitts!) transferring them to a wire cooling rack to finish cooling.
Serve and enjoy for breakfast, dessert, or anytime! Store any leftover muffins in an airtight bag or container on the counter for 3-4 days.