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 Together the Dry Ingredients: In a large mixing 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. Use a whisk to thoroughly combine these ingredients, making sure there are no lumps and the spices are evenly distributed. This step is crucial for an even rise and flavor.

3. Combine the Wet Ingredients: In a separate medium bowl, whisk together the large egg, 1/2 cup milk, 1/4 cup vegetable oil, and 1 teaspoon vanilla extract until well combined. The mixture should be smooth and uniform.

4. Gently Combine Wet and Dry Mixtures: Pour the wet ingredients into the bowl with the dry ingredients. Stir with a spoon or spatula until just combined. Be careful not to overmix; a few lumps are perfectly fine. Overmixing can lead to tough muffins. Once mostly combined, gently fold in the 1 cup of chopped apples.

5. Prepare the Streusel Topping: In a small bowl, combine the 1/4 cup all-purpose flour, 2 tablespoons granulated sugar, and 1/2 teaspoon ground cinnamon. Add the 1 tablespoon of 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. This creates the signature crumbly texture of streusel.

6. Fill Muffin Cups and Add Topping: Divide the muffin batter evenly among the 12 prepared muffin cups, filling each about two-thirds full. Sprinkle a generous amount of the streusel topping over the top of each muffin. Don’t press it down; just let it sit on top.

7. Bake Until Golden: Bake for 20-25 minutes, or until the muffins are golden brown and a wooden skewer or toothpick inserted into the center of a muffin comes out clean. The streusel should be lightly toasted and fragrant.

8. Cool Before Serving: 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 becoming soggy and makes them easier to handle.