Heat oven to 350 degrees. Spray muffin tin. In medium bowl, whisk four, sugar, baking powder, baking soda, & salt. Make a well in center of flour mixture. In well, mix together butter, bananas, eggs and vanilla. Stir to incorporate flour mixture (do not overmix). Dividing evenly, spoon mixture into muffin cups. With a teasp and a toothpick swirl in Nutella (1/2 tsp each for mini muffins or 1 tsp each for regular size).
Bake until tootpick inserted in center comes out clean (12-15 min for mini & 17-20 min for regular).
Remove muffins and cool on wire rack.
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