Prep: Preheat your oven to 180°C (350°F). Line a 12-cup muffin pan with paper liners.
Wet Ingredients: In a large mixing bowl, whisk together the melted butter, granulated sugar, brown sugar, Greek yogurt, milk, eggs, vanilla extract, cinnamon, and kosher salt until smooth.
Fold Apples: Gently fold the peeled, diced apple chunks into the batter until evenly distributed.
Portion: Scoop the batter evenly into the muffin liners, filling each cup about ¾ to ⅞ full for tall bakery caps.
Make the crumble & bake
Make Crumble Topping: In a medium bowl, combine the flour, brown sugar, cinnamon, kosher salt, and melted butter. Stir with a fork until crumbly.
Top: Generously scatter the crumble over each muffin, pressing gently so it sticks to the batter.
Bake: Bake for 18 to 20 minutes, or until a toothpick inserted into the center comes out clean with just a few moist crumbs. Cool in the pan for 15 minutes before glazing.
Make the simple glaze
Glaze: In a small bowl, whisk together the powdered sugar, milk, and vanilla extract until smooth and silky. Drizzle over the warm or cooled muffins before serving.
Notes
Flour: Weighing your flour on a kitchen scale gives the best results. If using measuring cups, spoon the flour into the cup and level it off with a knife instead of scooping directly from the bag.
Apples: Firm varieties like Granny Smith, Honeycrisp, or Pink Lady work best. Peel and dice them into small ½-inch pieces so they distribute evenly.
Room Temp Ingredients: Pull the eggs, milk, and yogurt out of the fridge early so they're at room temperature. Cold ingredients will cause the melted butter to seize and clump.
Crumble Topping: Press the crumble gently into the top of the batter before baking so it sticks to the muffins as they rise.
Reheating: Warm stored muffins in a 350°F (175°C) toaster oven for 3 to 5 minutes to crisp the crumble back up. Skip the microwave so the topping doesn't get soft.