Heat the oven to 425°F. Line a 13×18-inch rimmed baking sheet with parchment paper; generously coat the parchment and sides of the pan with nonstick cooking spray.
Melt the butter in a small bowl; set aside to cool slightly. Mash the bananas in a medium bowl (you should have about 2 cups mashed). Add the cottage cheese, milk, eggs, maple syrup, melted butter, and vanilla and whisk to combine.
In a large bowl, whisk the flour, cinnamon, baking powder, baking soda, and salt. Fold in the banana mixture and stir until just combined. Fold in the chopped prunes.
Scrape the batter into the prepared baking sheet and smooth with a spatula. Slice the extra banana, if using, and sprinkle over the batter along with the chocolate chips.
Bake until puffed and a toothpick comes out clean, 10 to 12 minutes.
Meanwhile, make the syrup: add the prune juice to a small pot set over medium heat. Bring to a boil, then reduce the heat and simmer until reduced to the consistency of maple syrup.
Slice into 12 pieces and serve with more butter and prune syrup.
