Prep your pan and oven: Heat the oven to 350°F (175°C). Line a 12-cup muffin pan with paper liners or grease it lightly.
Mash the bananas: In a large bowl, mash 2–3 very ripe bananas until mostly smooth.
A few small lumps are fine.
Add wet ingredients: Whisk in 1/2 cup melted butter (or oil), 1/2 cup brown sugar, 1/4 cup granulated sugar, 2 large eggs, 1 teaspoon vanilla, and 1/3 cup Greek yogurt or sour cream. Mix until combined.
Combine dry ingredients: In a separate bowl, whisk 1 3/4 cups all-purpose flour, 1 teaspoon baking powder, 1/2 teaspoon baking soda, 1/2 teaspoon salt, and 1/2 teaspoon cinnamon.
Bring it together: Add the dry ingredients to the wet ingredients. Stir gently with a spatula until just combined. Do not overmix.
A few streaks of flour are okay.
Fold in the chocolate chips: Add 3/4–1 cup chocolate chips and gently fold to distribute. If using nuts, fold in 1/2 cup now.
Fill the cups: Divide the batter evenly among the muffin cups, filling each about 3/4 full. Sprinkle a few extra chips or a pinch of coarse sugar on top if you like.
Bake: Bake for 18–22 minutes, or until the tops are set and a toothpick inserted into the center comes out with a few moist crumbs (no wet batter).
Cool: Let the muffins rest in the pan for 5 minutes, then transfer to a wire rack to cool.
Enjoy warm while the chocolate is melty.