Preheat the oven to 375°F (190°C). Line a 12-cup muffin pan with tulip-style or parchment cupcake liners. These liners work best because they help the muffins bake tall without overflowing.

In a large mixing bowl, mash the bananas until mostly smooth. A few small lumps are perfectly fine.

Whisk in the melted (and cooled) butter, oil, brown sugar, and granulated sugar until the mixture looks smooth and glossy.

Add the eggs, Greek yogurt, and vanilla extract. Whisk again until fully combined.

Sprinkle the flour, baking soda, baking powder, salt, and cinnamon evenly over the wet ingredients.

Using a spatula, gently fold everything together just until no dry flour remains.

Do not overmix to keep the muffins soft and tender.

In a small bowl, stir together the brown sugar, cinnamon, and melted butter for the cinnamon swirl.

Spoon the batter into the prepared liners, filling each one about halfway. Add a spoonful of the cinnamon swirl mixture to each muffin, then divide the remaining batter evenly on top.

Use a knife or skewer to gently swirl the cinnamon mixture into the batter. Just a couple of turns is enough.

Bake for 18 to 22 minutes, or until the tops are set and a toothpick inserted into the center comes out clean or with just a few moist crumbs.

Remove the muffins from the oven. While they are still hot, brush the tops with melted butter and sprinkle generously with cinnamon sugar.

Let cool in the pan for 5 minutes, then transfer to a wire rack. Serve warm or at room temperature.