Preheat oven to 375° F. Spray a muffin tin with cooking spray. (I use a nonstick muffin pan and lightly spray it with coconut oil spray.)
Place the butter in a large mixing bowl and melt in the microwave. (Or you can melt it in a pan on the stove and then transfer to a large mixing bowl.)
⅓ cup (75g) unsalted butter
To the bowl with the melted butter, add the mashed banana and brown sugar. Whisk until well combined. Whisk in the egg and vanilla.
1 ¼ cups (295g) mashed ripe banana, ½ cup (100g) packed light brown sugar, 1 large egg, 1 ½ teaspoons vanilla extract
Add the cinnamon, baking soda, baking powder and salt to the bowl and whisk until well combined.
1 teaspoon ground cinnamon, 1 teaspoon baking soda, 1 teaspoon baking powder, ½ teaspoon salt
Add the flour and mix it in with a rubber spatula until just combined. Then gently mix in the chocolate chips; do not over-mix the batter.
1 ½ cups (180g) all-purpose flour, ¾ cup (140g) chocolate chips
Scoop the batter into the greased muffin tin, filling each well about ⅔ full. Optional: Press a few extra chocolate chips into the tops of the muffins.
Bake at 375° F for 15-18 minutes, or until a toothpick inserted into the center of a muffin comes out clean (except for possibly melty chocolate).
Let muffins cool in the pan for 10 minutes and then remove to a wire rack. Serve warm or at room temperature.