Begin by preheating your oven to about 375°F (190°C), which is a common baking temperature for fruit muffins and helps them rise quickly while staying moist inside. Lightly grease a 12-cup muffin pan with nonstick spray or line the cups with paper liners to prevent sticking and make cleanup easy.​

Prepare the apple next so it is ready when you mix the wet ingredients: peel the McIntosh apple, then grate it on the coarse side of a box grater, working around the core and discarding seeds and tough center pieces. Lightly squeeze the grated apple in your hands only if it seems extremely juicy, but do not wring it dry because the natural moisture helps keep the muffins tender.​

In a large bowl, whisk together the all-purpose flour, baking powder, pumpkin pie spice, and salt until everything looks evenly blended and no streaks of spice remain. Whisking the dry ingredients like this distributes the leavening and seasoning so that every muffin bakes evenly and carries the same warm spice flavor.​

Make a shallow well in the center of the dry mixture with your whisk or a spoon, which will help when you pour in the wet ingredients later. Keeping dry and wet components separate until the last moment reduces the risk of overmixing and preserves a light crumb in the finished muffins.​

In a separate medium bowl, add the granulated sugar, egg, unsweetened applesauce, vegetable oil, and whole milk. Whisk these together until the egg is fully broken up and the mixture looks smooth and slightly creamy, with the sugar beginning to dissolve into the liquid base.​

Now stir in the grated McIntosh apple, folding it through so that the shreds are evenly suspended in the wet mixture rather than clumping in one spot. The combination of applesauce, oil, milk, and fresh apple creates a balance of moisture and richness, allowing the muffins to stay soft without needing a large amount of fat.​

Pour the wet mixture into the bowl with the dry ingredients, aiming mostly for the center of the flour well. Using a spatula or wooden spoon, gently fold the mixture together, scraping from the bottom of the bowl and turning the batter over itself just until you no longer see obvious pockets of dry flour.​

The batter should look thick, slightly lumpy, and flecked with apple strands; stop mixing as soon as it reaches this stage because overworking it will develop gluten and create tough muffins. Use a spoon or small scoop to divide the batter evenly among the prepared muffin cups, filling each about two-thirds to three-quarters full to allow room for rising without overflowing.​

Place the muffin pan on the center rack of the preheated oven so the heat circulates evenly around the pan. Bake for about 18–22 minutes, watching for the tops to dome and turn a light golden color around the edges as the muffins set.​

To test doneness, insert a toothpick into the center of a muffin; if it comes out clean or with just a few moist crumbs, the muffins are ready, but if you see wet batter, continue baking for a couple more minutes and test again. Remove the pan from the oven and let the muffins cool in the tin for 5 minutes to firm up before carefully transferring them to a wire rack.​

Allow the muffins to cool on the rack until they are just warm to the touch if you plan to serve them right away; the aroma of pumpkin pie spice and baked apple will be strongest at this stage. These muffins pair nicely with coffee, tea, or a glass of cold milk, and their gentle sweetness makes them suitable for breakfast or an afternoon snack.