1cup (250g)banana puree,made from very ripe bananas
1/2cup (120g)sour cream or plain yogurt,whole-milk or lowfat
1teaspoonvanilla extract
1/2-3/4cup (80-120g)chocolate chips
1 1/2tablespoonscoarse sugar,such as turbinado (optional)
Preheat the oven to 350ºF (180ºC.) Butter a 9-inch (23cm) loaf pan and line the bottom with a sheet of parchment paper.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and sugars. Make a well in the center of the dry ingredients and add the egg, egg white, banana puree, sour cream or yogurt, and vanilla. Stir not too vigorously with a spatula until the ingredients are almost completely incorporated (don't overmix), then fold in the chocolate chips.
Scrape the batter into the prepared loaf pan. Sprinkle the sugar evenly over the top and bake until a toothpick inserted into the center comes out relatively clean, with just a few crumbs sticking to it, about 45 minutes. Remove the cake from the oven and let cool on a wire rack.
Notes
Storage: The cake will keep up to 5 days at room temperature, if well-wrapped. It can be frozen for up to 3 months.