Preheat oven to 375 degrees F. Line a 12-muffin pan with muffin or cupcake liners.
Make the streusel topping: In a bowl, whisk the streusel topping’s dry ingredients (flour, brown sugar, granulated sugar, cinnamon, and salt). Then, gradually drizzle in the melted and cooled butter, whisking with a fork to form moist crumbles. Tip: Make sure the melted butter is cooled. Don’t overmix, otherwise it will turn into a paste.
Mix dry muffin ingredients: In a bowl, whisk flour, granulated sugar, brown sugar, baking soda, baking powder, salt, and cinnamon until evenly combined. Set aside.
Mix wet muffin ingredients: In a separate large bowl, mix the melted butter, Greek yogurt, milk, eggs, and vanilla extract until well combined. Set aside.
Add the dry ingredients into the wet ingredients, and mix until just combined and no pockets of flour remain. Do not overmix. The batter will be thick.
Using a rubber spatula, fold in the diced apples into the batter. Set aside.
Evenly fill the prepared muffin pan with the batter, all the way to the top (but don’t overfill). Sprinkle the prepared streusel on top, gently pressing it down just slightly. Tip 1: Filling all the way to the top will help the muffins rise tall. It won’t spill over since the batter is thick. Tip 2: I use a large cookie scoop to add the batter into the pan. It’s convenient, easy, and it fits just the right amount of batter.
Bake at 375 degrees F for 22-25 minutes or until a toothpick inserted in the middle comes out clean. Remove from oven and let cool for a few minutes before serving. Enjoy! Tip: Do ensure the muffins rise properly, avoid opening the oven as they bake - sudden changes in temperature can cause baked goods to collapse.