Start by milling the flour for both the crumb topping and the muffins. Freshly milled flour adds flavor and improves texture.

Add the flour, sugar, and cinnamon to a small bowl. Stir briefly to combine. Pour in the melted butter, then use a fork to mix until a coarse, crumbly texture forms. The mixture should look rustic, not smooth.

Place the crumb topping in the refrigerator while you prepare the muffin batter. Chilling helps the topping hold its shape during baking and creates a better bakery-style finish.

Preheat your oven to 425°F so it is fully hot when the muffins go in.

Line a 12-count muffin tin with liners. Tulip-style liners work especially well for large, bakery-style muffins and help keep the crumb topping from sliding off as the muffins bake.

In a large mixing bowl, add the flour, baking powder, baking soda, salt, and cinnamon. Whisk to evenly distribute the dry ingredients.

Gently fold in the diced apples just until coated with flour. This helps prevent them from sinking to the bottom while baking.

In a separate bowl, whisk together the sugars and oil. Add the eggs, sourdough starter, milk, and vanilla. Mix until smooth and fully combined.

Pour the wet ingredients into the bowl with the dry ingredients. Stir gently until just combined. Do not overmix. A slightly lumpy batter is normal and helps keep the muffins tender.

Scoop the batter evenly into the prepared muffin tin. Fill each liner generously for tall, bakery-style muffins. If using tulip liners I fill with batter to the top of the pan.

Remove the crumb topping from the refrigerator and sprinkle it evenly over each muffin.

Bake the muffins at 425°F for 5 minutes, then reduce the oven temperature to 350°F without opening the door. Continue baking for 15-20 minutes, or until the tops turn light golden brown and a toothpick inserted in the center comes out clean.

Remove the muffins from the oven and let them cool in the pan for 10 minutes. Transfer them to a wire rack to finish cooling.