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Preheat oven to 220C/425F. Butter 12 muffin cups or line them with liner papers.
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The streusel: Place all ingredients in a food processor and pulse until mixture becomes crumbly and unevenly shaped pieces are formed. Alternatively, you can crumble the ingredients in a bowl using your fingertips, a pastry cutter, or a fork. Cover and place in the refrigerator.
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The muffins: In a large bowl, sift together flour, baking powder, baking soda, salt, cinnamon, and nutmeg. Add granulated sugar and brown sugar, and mix.
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In a medium bowl, whisk together eggs, buttermilk, melted butter, and vanilla extract until combined. Pour the wet ingredients into the dry ingredients and stir with a wooden spoon or rubber spatula just until combined. Do not over mix. The batter should be thick and lumpy. Fold in apples.
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Divide the batter evenly between muffin cups, filling almost fully to the top. Sprinkle the streusel into the top of each. Bake for 3 minutes and then reduce the oven temperature to 180C/350F and continue to bake for an additional 12-17 minutes or until a toothpick inserted into the center of a muffin comes out clean. Cool muffins in the pan on a wire rack for 10 minutes. Remove from pan and set on wire rack to cool completely.
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Muffins are best the same day they are made, but can be stored in an airtight container at room temperature for 2-3 days. Muffins can also be frozen for up to 2 months.