Preheat the oven to 350 °F. Butter and flour (or butter and line with parchment) a 9"x 5" inch loaf pan.
Mash 3 very ripe medium bananas with a fork, leaving them a little chunky. Add 3 oz full-fat plain Greek yogurt, 2 oz vegetable oil, 1 oz unsalted butter, 2 large eggs, 1 teaspoon vanilla extract, and ½ teaspoon lemon extract to the bananas. Mix to combine and set aside.
Toss 6 oz mini chocolate chips with 2 teaspoons of water to moisten. Sprinkle a teaspoon of all purpose flour over the chips and toss, set aside.
Combine 9 oz all-purpose flour, 1 ¼ oz buckwheat flour, 6 oz granulated sugar, 1 teaspoon baking powder, ½ teaspoon baking soda, ½ teaspoon salt, ½ teaspoon ground cinnamon and ¼ teaspoon nutmeg in a large bowl. Whisk the dry ingredients to combine. Add the banana mixture and stir until about 2/3 of the flour is incorporated. Fold in the chocolate chips and mix until all the flour is incorporated.
Pour the batter into the prepared pan. Sprinkle additional chocolate chips on top if you'd like.
Bake until golden brown and a toothpick inserted in the middle comes out clean, about 50-60 minutes. Cool for 10 minutes in the pan, then turn out onto a cooling rack to finish cooling.