Preheat the oven to 350° F. Generously butter or grease with olive oil a 10-inch enameled or well-seasoned cast-iron skillet, or a 9x5-inch loaf pan. (If your skillet isn't well-seasoned, you can line the bottom with a round of parchment paper.)

In a large bowl, whisk together the flour, cocoa, brown sugar, baking soda, and salt.

In a separate bowl, mix together the olive oil, eggs, mashed banana, sour cream, and vanilla. Pour the banana mixture into the flour mixture and fold with a spatula until just combined. Fold in the chocolate and pecans (if using), again just until combined. Scrape the batter into the prepared skillet.

Bake until golden brown and a toothpick or knife inserted into the center of the bread comes out clean, about 28 to 35 minutes (start checking at about 22 minutes for doneness). Let cool in skillet for about 20 to 30 minutes, and serve warm or at room temperature, straight from the pan. (If using a loaf pan, the baking time will be closer to 55 to 65 minutes. Let cool in the loaf pan for about 10 to 15 minutes, then run a knife around the edge of the pan and invert it onto a cooling rack.)