Step 1: Prepare the Oven and Muffin Tin
Set the oven temperature to 375°F, or 190°C. Place paper liners inside a 12-cup muffin tray.
Step 2: Make the Crumble Topping (if using)
Put the cinnamon, brown sugar, and flour in a small bowl. Add the chilled butter and chop with your fingertips or a pastry cutter until coarse, sandy crumbs form. Put aside.
Step 3: Combine Dry Ingredients
Mix the flour, baking powder, nutmeg, cinnamon, baking powder, and both sugars in a big bowl.
Step 4: Combine Wet Ingredients
Whisk the milk, melted butter, vanilla extract, and softly beaten eggs in a different medium dish.
Step 5: Mix Wet and Dry Ingredients
Mix the dry ingredients with the wet ones. Using a spatula, stir just until blended. Avoid overmixing; this will result in a thick, lumpy batter that is ideal for soft muffins.
Step 6: Fold in Apples
Add the finely chopped apples to the batter and fold gently.
Step 7: Fill the Muffin Tin
Evenly distribute the batter among the 12 muffin tins. The cups are going to be filled.
Step 8: Add the Topping
Top each muffin with a large amount of crumble topping, if using.
Step 9: Bake
A toothpick put into the middle of a muffin should come out clean after 20 to 25 minutes of baking, or until the tops are golden brown.
Step 10: Cool
After a few minutes of cooling in the muffin tray, move the muffins to a wire rack to finish cooling.