1. Prepare Your Oven and Muffin Tin: Start by preheating your oven to 375 F (190 C). Line a 12-cup muffin tin with paper liners or grease it thoroughly with non-stick spray. This ensures your muffins won’t stick and makes for easy cleanup.

2. Whisk Dry Muffin Ingredients: In a large bowl, combine the 1 1/2 cups all-purpose flour, 1/2 cup granulated sugar, 2 teaspoons baking powder, 1/2 teaspoon ground cinnamon, and 1/4 teaspoon salt. Whisk these ingredients together until they are well combined and there are no lumps.

3. Combine Wet Muffin Ingredients: In a separate medium bowl, whisk together the large egg, 1/2 cup milk, 1/4 cup melted and cooled unsalted butter, and 1 teaspoon vanilla extract until smooth.

4. Mix Muffin Batter and Add Apples: Pour the wet ingredients into the dry ingredients. Stir gently with a spatula until just combined. Be careful not to overmix; a few lumps are perfectly fine. Gently fold in the finely diced apple until evenly distributed throughout the batter.

5. Prepare the Crumb Topping: In a small bowl, combine the 1/2 cup all-purpose flour, 1/4 cup packed light brown sugar, 1/4 cup granulated sugar, and 1/4 teaspoon ground cinnamon. Add the 1/4 cup cold unsalted butter, cut into small pieces. Use your fingertips or a pastry blender to cut the butter into the dry ingredients until the mixture resembles coarse crumbs.

6. Assemble and Bake the Muffins: Divide the muffin batter evenly among the 12 prepared muffin cups. Sprinkle a generous amount of the crumb topping over the top of each muffin. Bake for 20-25 minutes, or until a wooden skewer inserted into the center of a muffin comes out clean and the tops are golden brown and the crumb topping is slightly crisp.

7. Cool and Serve: Once baked, remove the muffin tin from the oven and let the muffins cool in the tin for 5 minutes before transferring them to a wire rack to cool completely. This helps prevent them from getting soggy on the bottom. Serve warm or at room temperature.