Soak both the fresh and dried cherries in the port wine for at least 3 hours (overnight is best), making sure the cherries are completely covered with liquid.
Preheat the oven to 350F. Line a 9-inch x 9-inch baking pan with parchment paper and coat it with cooking spray.
Mash the butter with the banana. Add the eggs and water. Whisk until smooth.
In a separate bowl, combine the flour, sugar, cocoa powder, dark chocolate, and baking powder. Slowly sift the dry ingredients into the butter-banana mixture, stir- ring to incorporate after each addition. Mix until the batter is smooth and there are no lumps.
Gently fold in the port-soaked cherries, reserving a few to garnish and reserving any leftover liquid for another use. Stir to incorporate.
Pour the batter into the pan. Bake 40 minutes, or until a toothpick inserted in the center comes out clean. Serve garnished with the reserved cherries.