Mix the dry ingredients. Add the olive olive oil, eggs, milk, yogurt and vanilla extract. add the mashed bananas and sweet potato puree last. Then add the nuts last. You are looking for a batter with slightly moist texture. Separate about 1/3 of the batter. Stir in the cocoa powder in that part. If you love chocolate taste you can also add 2 table spoons of cocoa to the whole mixture without separating it. Grease a 8x3 loaf pan. Add haf of the "non cocoa" batter. Add the cocoa part on top in order to form a middle layer. Add the rest of the batter. Bake on 350 degrees for 60 minutes. The time may vary depending on the oven. Let cool before cutting.