One Banana Muffins (super moist!)

In a medium-sized, heat-safe mixing bowl, melt unsalted ¼ cup unsalted butter (i.e. ½ stick) in microwave for 20-40 seconds, or until fully melted.  
I recommend placing a paper towel on top of the bowl to prevent splatter.

Add the 1 large ripe banana (i.e. ½ cup)[/adjustable] to the bowl of melted butter and mash with a fork until broken up. You want to make sure you break up any large pieces or you will have chunks banana in your bread and it can cause it to bake weirdly.

Add in ¼ cup light brown sugar, and ¼ cup granulated sugar, then stir until combined. 
NOTE: To achieve the dense and moist texture we’re going for you MUST mix by hand! Using an electric mixer will whip too much air into the bread and affect the texture.

Add in the 1 large egg, ¼ cup sour cream, and 1 teaspoon vanilla extract, then stir until just combined. Once mixed, add in the ½ teaspoon baking soda, ¼ teaspoon baking powder, and ¼ teaspoon cinnamon, and ¼ teaspoon salt, then mix until just combined.

Lastly, mix in ½ tablespoon cornstarch, and ¾ cup all-purpose flour and stir until just combined. You do not want to mix this for any longer than it takes for the flour to disappear or it will become dense and chewy. Do not overmix!

Line a standard-sized cupcake pan with 7 muffin liners (stagger them every other vs. putting them all on one side; this will help the air flow.
Divide batter amongst the muffin liners (max 4 tablespoons of batter each).

Bake at 350°F for 22-24 minutes, or until a toothpick comes out clean.

Allow to cool, then enjoy!
If not eating right away, place fully-cooled muffins in an airtight container or plastic bag with a paper towel to help absorb excess moisture. Wrapping them in plastic while they're still warm will trap the steam and could make your muffins mushy.