Preheat the oven to 350°F (175°C). Line a 12-cup muffin pan with liners (lightly spray for extra nonstick, if desired).
Wet ingredients: In a medium bowl, whisk together milk, oil, egg, and vanilla extract. Set aside.
Dry ingredients: In a large bowl, whisk flour, baking powder, sugar, and salt.
Apples: Toss the chopped apples in the dry mixture to coat (this prevents them from sinking).
Pour the wet ingredients into the dry and mix gently until just combined; do not overmix. Divide the batter evenly between the muffin cups, filling about ¾ full.
Crumble topping: In a small bowl, combine cold butter, flour, brown sugar, and cinnamon. Rub together with your fingertips until crumbly.
Sprinkle crumble generously over each muffin.
Bake for 22–28 minutes, or until a toothpick inserted in the center comes out clean.
Cool slightly in the pan, then transfer to a wire rack. Serve warm or at room temperature.
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Notes
Apples: Use firm, slightly tart varieties like Granny Smith, Honeycrisp, or Pink Lady. Softer apples may break down more during baking.
Texture tip: Coating apples in flour helps distribute them evenly and keeps them from sinking.
Don’t overmix: Stir only until combined; overmixing leads to dense muffins.
Add-ins: Stir in a handful of chopped nuts (pecans or walnuts) or raisins for extra flavor.
Spice boost: Add ½ teaspoon cinnamon or apple pie spice to the muffin batter for more warmth.
Oil: Vegetable oil, canola oil, or sunflower oil all work well in this recipe. Oil keeps the muffins extra moist and tender. If you prefer a richer flavor, you can swap the oil for melted butter instead.
Make it whole grain: Swap half of the flour with whole-wheat flour for a heartier muffin.
Storage: Store muffins in an airtight container at room temperature for up to 2 days, or refrigerate up to 5 days. Warm before serving.
Freezing: Wrap cooled muffins individually and freeze up to 2 months. Thaw overnight or reheat briefly in the oven.