1. 1. Prepare your muffin tin and oven: Preheat your oven to 375 F (190 C). Line a 12-cup standard muffin tin with paper liners or grease it well with non-stick spray. This ensures your muffins won’t stick and makes for easy cleanup.

2. 2. Make the streusel topping: In a small bowl, combine 1/4 cup all-purpose flour, 2 tablespoons granulated sugar, 1 tablespoon brown sugar, and 1/4 teaspoon ground cinnamon. Add the 2 tablespoons of cold, diced butter. Use your fingertips or a pastry blender to cut the butter into the dry ingredients until the mixture resembles coarse crumbs. Set aside.

3. 3. Combine dry ingredients for the muffins: In a large mixing bowl, whisk together 1 1/2 cups all-purpose flour, 1/2 cup granulated sugar, 2 teaspoons baking powder, 1/2 teaspoon ground cinnamon, 1/4 teaspoon ground nutmeg, and 1/4 teaspoon salt. Make sure there are no lumps and everything is evenly distributed.

4. 4. Mix wet ingredients: In a separate medium bowl, lightly beat the large egg. Then, whisk in 1/2 cup milk, 1/4 cup vegetable oil (or melted butter), and 1 teaspoon vanilla extract until well combined.

5. 5. Combine wet and dry mixtures: Pour the wet ingredients into the bowl with the dry ingredients. Stir gently with a spatula or wooden spoon until just combined. Be careful not to overmix; a few lumps are perfectly fine and help keep the muffins tender. Gently fold in the finely diced apple pieces.

6. 6. Fill the muffin cups and add streusel: Divide the muffin batter evenly among the 12 prepared muffin cups. Each cup should be about two-thirds full. Sprinkle a generous amount of the prepared streusel topping over the batter in each cup.

7. 7. Bake the muffins: Place the muffin tin in the preheated oven and bake for 20-25 minutes, or until the tops are golden brown and a wooden skewer or toothpick inserted into the center of a muffin comes out clean.

8. 8. 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. Enjoy them warm or at room temperature.