Preheat oven to 350°F. Grease a muffin tin with non-stick cooking spray or line with muffin liners.
Add the bananas to a bowl with the warm, melted butter and mash using a fork or potato masher.
Add the oil, buttermilk, beaten eggs, and vanilla. Mix well to combine.
In a second large bowl, whisk together the flour, sugar, brown sugar, baking soda, salt, cinnamon, and nutmeg until combined.
Pour the wet ingredients into the dry ingredients and mix together with a wooden spoon until combined (the mixture will be thick).
Divide batter between 12 muffin cups and bake for 20-25 minutes or until a cake tester comes out clean.
Remove from the oven and allow to cool for five minutes before removing from the pan. Serve warm or cooled.
Store remaining muffins covered at room temperature for up to 3 days.
Be sure to follow the steps of the recipeclosely. Mixing the dry and wet ingredients well before combining them helps to avoid overmixing (which can create a dense, tough muffin).
Measure the flour correctly by using a measuring cup for dry measurements, then level off the top of the cup once the flour is scooped. This will ensure you’re not adding extra, which could result in a dry muffin.