Preheat the oven to 350 degrees F and spray the muffin pan with nonstick spray.
Put the raisins and dates in a medium bowl and cover with very hot water. Let soak until the fruit is very soft, about 25 minutes. Drain and then puree until smooth in a food processor, scraping the bowl frequently.
Whisk the flour, baking powder, cinnamon and salt together in a medium bowl.
Beat the butter in a large bowl with an electric mixer on medium-high speed until smooth. Add the raisin-date mixture and beat until incorporated and smooth. Add the egg and egg yolk, vanilla and orange zest and beat until incorporated. Lower the mixer speed to medium-low and add half the flour mixture, then all of the milk and then the remaining flour mixture, stopping to scrape down the sides of the bowl as needed (it's ok if the batter has some lumps). Fold the banana chunks into the batter.
Divide the muffin batter evenly among the mini muffins cups. Dampen your hands and smooth the tops of each mound of batter. Top each with a thin slice of banana. Bake until the muffins are golden and bounce back when pressed, about 18 minutes. Let cool in the pan for a few minutes and then transfer to a rack to cool completely. Store at room temperature in an airtight container for up to 3 days.
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