I found it easier to grate the chocolate in the food processor than the hand grater. I also sometimes substitute a bag of mini chocolate chips for the grated chocolate if you like a real "chocolatey" taste!
Adjust the oven rack to the lower-middle position and preheat the oven to 350°.
Grease the bottom and sides of a 9" x 5" loaf pan, dust with flour, tapping out the excess.
Whisk the flour, sugar, chocolate, baking soda, and salt together in a large bowl; set aside.
Mix the mashed bananas, yogurt, eggs, butter, and vanilla with a wooden spoon in a medium bowl.
Lightly fold the banana mixture into the dry ingredients with a rubber spatula until just combined and the batter looks thick and chunky.
Scrape the batter into the prepared loaf pan.
Bake until the loaf is golden brown and a toothpick inserted in the center comes out clean, about 55-60 minutes. Cool in pan for 5 minutes, then transfer to a wire rack.
Serve warm or at room temperature. (The bread can be wrapped in plastic wrap and stored at room temperature for up to 3 days.)