Preheat oven to 425 degrees and grease a 12-cavity muffin pan or line with paper or silicone liners.
Stir together 1/4 cup sugar and the cinnamon and set aside.
In a medium bowl whisk together flour, baking powder, baking soda, and salt.
In a large bowl mix together mashed bananas, Greek yogurt or sour cream, brown sugar, 1/3 cup granulated sugar, melted butter, egg, vanilla, and milk.
Add dry ingredients to wet ingredients and stir until combined.
Fill muffin liners 2/3 full. Top with 1/2 teaspoon cinnamon-sugar mixture. Use a toothpick to swirl the cinnamon-sugar into the muffin batter (I do a figure-8 with my toothpick several times in different directions).
Fill liners the rest of the way full, dividing remaining batter evenly between all of the muffins. Top with another 1/2 teaspoon cinnamon-sugar and repeat swirling technique.
Bake for 5 minutes, then set temperature to 350 and continue to bake 10-15 minutes longer until a toothpick inserted in the center of the muffins comes out clean.
Allow to cool before serving or storing in airtight container at room temperature up to 5 days OR wrapped individually and chilled up to 2 weeks.