Small-batch Chocolate Banana Muffins (One-banana, One-bowl)

muffin liners

Fine-mesh strainer

medium-to-large over-ripe banana

unsalted butter (melted)

brown sugar

large egg white

vanilla extract

all-purpose flour (measured by weight or using the spoon and sweep method*)

unsweetened cocoa powder

baking soda

salt

semi-sweet chocolate (chopped or 1/3 cup chocolate chips, plus more for topping)