Preheat the oven to 350 degrees F. Spray a 9- by 5-inch loaf pan with cooking spray.
Put the raisins and dates in a medium bowl, cover with very hot water and let soak until the fruit is very soft, about 25 minutes. Drain, then puree until smooth in a food processor, scraping the bowl frequently.
Whisk together the flour, baking powder, cinnamon and salt 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 zest, and beat until incorporated. Lower the mixer speed to medium-low, and add half the flour mixture, then the milk, then the remaining flour mixture, stopping to scrape down the side of the bowl as needed (it's OK if there are some lumps). Fold in the bananas.
Pour the batter into the prepared pan, and smooth the top with a spatula or the back of a spoon. Bake until the bread is golden brown and crisp and a toothpick inserted in the center comes out clean, about 1 hour. Run a knife around the edges, and let cool completely in the pan on a rack. The bread can be stored at room temperature in an airtight container for up to 3 days.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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