Prepare your muffin tin with muffin liners and pre-heat oven to 175C / 347F (no fan).
In a large bowl, cream the room temperature butter and sugar with an electric hand mixer for a few minutes until light and fluffy.
After, slowly pour in the oil while continuing to whip the mixture. Add the eggs one by one, and then pour in the vanilla extract, followed by the room-temperature milk. Keep whipping the batter for a few seconds. Finally, mix the banana puree.
Sift in the dry ingredients (flour, baking powder, salt), then switch to a rubber spatula and fold the dry ingredients in. Do not overmix at this point.
In a small bowl, mix vinegar with baking soda, it will be super bubbly. Immediately fold the vinegar mixture into the muffin batter just until combined.
Pipe or spoon the muffin batter into the cavities of your muffin tin equally until about ¾ of each. Mix into the batter about one tablespoon of Nutella per cavity, then place one more tablespoon of Nutella on top of each.
Bake the muffins for about 22 minutes or until a toothpick inserted comes out clean. Do not overbake the muffins as they can dry out.
Serve the muffins fresh or store them at room temperature for 2-3 days in an airtight container.
Notes
INGREDIENT NOTES
All ingredients must be at room temperature to properly emulsify: eggs, butter, and milk.
The baking soda and vinegar mixture is my signature baking hack for making muffins and cupcakes super moist and fluffy; don't skip it.
You might be able to make more or fewer muffins with this recipe, depending on the size of your muffin tin.
A digital scale is required for a consistent, happy baking experience.
Do not overwork the muffin batter. After you mix the wet ingredients, switch to a rubber spatula to gently incorporate the dry ingredients.
Always pre-heat your oven and consider using a digital oven thermometer for the best results.
When whipping in the eggs, oil, and milk, make sure they are all at room temperature. Otherwise, the batter will break.
Also, make sure to add the eggs, oil, and milk slowly. Do not pour all the wet ingredients at once, as that can also break the muffin batter and cause it to curdle.
Baking these muffins takes 22 minutes in my oven; however, focus on the texture, not the time. Apply the toothpick check (it should come out clean) to avoid underbaked muffins. However, do not overbake them, as that would result in a dry texture.