Prepare the Pan: Preheat your oven to 350°F and grease a 9x5-inch loaf pan. Line it with parchment paper for easy removal.
Make the Cinnamon Swirl: In a small bowl, combine brown sugar and cinnamon. Set aside.
Prepare the Crumb Topping: In a medium bowl, mix flour, brown sugar, and cinnamon. Grate cold butter into the mixture, and use your hands to combine until it resembles coarse crumbs. Set aside.
Mix the Batter: In a separate bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon. In a large bowl, cream together softened butter and sugar until light and fluffy. Beat in the eggs, one at a time. Add Greek yogurt and vanilla, mixing until smooth. Gradually add the dry ingredients to the wet ingredients, stirring until just combined. Be careful not to overmix. Fold in the diced apples.
Assemble the Loaf: Pour half of the batter into the prepared pan. Sprinkle the cinnamon swirl mixture evenly over the top. Add the remaining batter and top it with the crumb mixture.
Bake: Bake for 50-60 minutes or until a toothpick inserted into the center comes out clean. If the loaf browns too quickly, cover it loosely with aluminum foil. Let the loaf cool in the pan for 10-15 minutes before transferring it to a wire rack.