If you do not have muscovado sugar in your larder, you may always substitute light brown sugar, or light brown sugar and some molasses.
Preheat the oven to 350 degrees Fahrenheit. Grease a standard-size loaf pan (approximately 9-½ by 5 inches), and line it with parchment paper.
In a small bowl, combine the flour, baking powder, and kosher salt. Whisk to blend.
In another bowl, mash the bananas with a fork. They should still be lumpy, not fully pureed. Stir in the vanilla.
In another bowl, beat the eggs lightly with a fork.
Using an electric mixer, beat the butter briefly, just to get it going, and then add the sugar. Continue beating until the mixture is light, fluffy, and the color of coffee with milk. Add the beaten eggs, and continue to beat. (If the mixture looks like it's curdling at any point, add a spoonful of the flour mixture.) Add the chocolate and the mashed bananas, and beat to mix. With the mixer on low, add the flour mixture. Beat to incorporate.
Scrape the batter into the prepared loaf pan, and smooth the top. Bake for about 50 minutes, or until a toothpick inserted into the center of the loaf comes out moist but clean. If there is any sign of wet batter, continue baking for a few more minutes, tenting the top with foil if it's browning too much.
Cool the finished loaf in the pan for 15 minutes. Loosen the sides with a thin knife, then carefully lift out the loaf with the parchment liner. Cool completely before slicing.
Yield: 1 loaf