Preheat oven to 220C/425F. Butter 12 muffin cups or line them with liner papers.

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.

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.

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.

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.

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.