Preheat your oven to 350°F (175°C). Grease and flour a 9x5 inch loaf pan to prevent sticking.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and ground cinnamon. Set aside.
In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy, about 2-3 minutes.
Add the eggs one at a time, mixing well after each addition. Include the vanilla extract and Greek yogurt; mix until well combined.
Gradually add the dry ingredients to the wet mixture, stirring until just combined. Be careful not to overmix.
Fold in the diced apples and chopped walnuts (if using) gently into the batter.
Pour the batter into the prepared loaf pan and smooth the top. In a small bowl, mix together the cinnamon sugar, then sprinkle it generously over the batter.
Bake in the preheated oven for 55-65 minutes, or until a toothpick inserted in the center comes out clean.
Once baked, remove from the oven and allow the bread to cool in the pan for 10 minutes. Then, transfer it to a wire rack to cool completely.
Notes
Slice the bread and serve it warm with a sprinkle of powdered sugar on top or a pat of butter for extra creaminess. Enjoy alongside a cup of tea or coffee for a delightful treat!