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1. In a medium bowl, combine the rolled oats and buttermilk, stir well, and let the mixture soak at room temperature so the oats soften and absorb the liquid.
1.5 cup old-fashioned rolled oats,
1.2 cup buttermilk
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2. Preheat the oven to 375 F (190 C). Line a 12-cup muffin tin with paper liners or lightly grease each cup.
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3. In a large bowl, mash the ripe bananas until mostly smooth, then whisk in the egg, brown sugar, granulated sugar, melted butter, and vanilla extract until well combined.
2 piece ripe bananas,
1 piece large egg,
0.5 cup light brown sugar, packed,
0.2 cup granulated sugar,
0.3 cup unsalted butter, melted and cooled slightly,
1.5 tsp vanilla extract
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4. Stir the soaked oat mixture into the banana mixture until evenly blended.
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5. In a separate bowl, whisk together the all-purpose flour, baking powder, baking soda, fine sea salt, and ground cinnamon until the leavening and spices are evenly distributed.
1.2 cup all-purpose flour,
1.5 tsp baking powder,
0.5 tsp baking soda,
0.5 tsp fine sea salt,
1 tsp ground cinnamon
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6. Add the dry ingredients to the wet mixture and gently fold just until no dry streaks of flour remain; if using, fold in the chopped walnuts, taking care not to overmix.
0.5 cup chopped walnuts (optional)
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7. Divide the batter evenly among the prepared muffin cups, filling each about three-quarters full, and sprinkle the tops with the extra rolled oats if using.
0.2 cup rolled oats (topping, optional)
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8. Bake until the muffins are risen, golden, and a toothpick inserted in the center comes out with a few moist crumbs, about 16 to 20 minutes.
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9. Let the muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely; once cool, store in an airtight container at room temperature for up to 3 days or freeze for longer storage.
