Preheat oven to 350F and line a 12-cup muffin tin with liners. (Note that these have a tendency to cling a bit to the liners, so I don’t know whether you want to lightly grease it, or use parchment liners.)
Cream together butter, sugar, and egg until fluffy. Add oil, Greek yogurt, mashed banana, and vanilla extract, and whisk again until blended. (If using oil only, cream together oil, sugar, and egg first, then add everything else.)
In a separate bowl, sift flour, baking soda, and salt together. Add dry ingredients to wet and stir gently until incorporated. The batter will be quite thick.
Divide batter evenly between liners. You should have just enough — scraping down the bowl should result in the liners filled about 2/3 full. (Which is good, because otherwise I’d be licking up so much raw batter.)
For the Nutella swirl, warm about two tablespoons of Nutella with a few drops of oil in the microwave for about 10 seconds. Stir until fluid, then drop small dots of Nutella over each cupcake, three or so each. Very gently run the very tip of a butter knife or a toothpick through each dot in a circle to form swirls. You can also use peanut butter, or omit the swirl entirely.
Bake at 350F for 12-15 minutes, or until tops are golden and spring back when touched, and a toothpick inserted comes out clean. Mine were done at 12 minutes exactly.