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1. Preheat oven to 425 degrees and line muffin tins with liners.
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2. Topping: Combine all ingredients until they form a crumbly mixture. Set aside.
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3. In a small bowl, toss apple pieces with cinnamon and brown sugar. See aside.
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4. In medium bowl, whisk together flour, baking powder, cream of tartar, cinnamon, nutmeg and salt.
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5. In a stand mixer, combine down sugar, sugar, and oil. Scrape the bowl down as needed. Add eggs one at a time until combined. Add in sour cream and vanilla extract and mix again. From this point on, no over-mixing!
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6. Slowly add in dry ingredients to the wet ingredients. Just before the flour is all combined, dump in your apple pieces coated in cinnamon/brown sugar and gently fold just until incorporated.
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7. Fill muffin tins super full (see photo) and bake for 5 minutes (in your preheated 425 oven) and then decrease temperature to 375 and bake for an additional 12-16 minutes.
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8. Let cool before glazing. Glaze: Whisk together milk with 1/2 cup powdered sugar. Add the additional powdered sugar if needed. I always glaze one muffin and wait a few minutes to see if I want the glaze any stiffer.