1. Preheat the oven to 350°F (180°C). Line a muffin tin with 12 paper liners and set aside.

2. Add the flour, baking powder, cinnamon, and salt to a large bowl and mix to combine. In a second bowl, whisk together yogurt, sunflower oil, maple syrup, vanilla extract, and the lightly beaten egg. Set both aside.

3. Peel the apples, remove the cores, and cut them into small cubes. Coarsely chop the walnuts. Coarsely chop the chocolate as well.

4. Add the yogurt mixture to the bowl with the flour and mix until just combined. Add the diced apples and fold in. Reserve about 2 tablespoons each of the chopped walnuts and chocolate and add the rest to the bowl and fold in.

5. Divide the batter between the prepared paper liners and sprinkle with the remaining chopped walnuts and chocolate. Bake the muffins for about 26-32 minutes or until a toothpick inserted into the center of a muffin comes out clean. Take out of the oven and let cool down completely on a wire rack.