Preheat oven to 375° F. Grease a standard 12-cup muffin pan or line with baking cups.
TO MAKE THE TOPPING:
In a small mixing bowl, stir together the brown sugar, gluten free flour blend, and cinnamon.
Add the butter and combine with a pastry blender, a fork, or your fingers until the mixture forms coarse crumbs.
Set aside while you make the muffins.
TO MAKE THE MUFFINS:
In a medium mixing bowl, whisk together the gluten free flour blend, baking soda, and salt.
In a large mixing bowl, stir together the egg, buttermilk, melted butter, vanilla extract, and brown sugar until combined.
Fold the flour mixture and apples into the buttermilk mixture until just combined.
Divide the batter evenly among the prepared muffin cups.
Sprinkle the topping over the tops of the muffins.
Bake 22 to 25 minutes, or until the tops of the muffins are golden brown and a wooden pick inserted into the center comes out clean.
Cool in the pan for 10 minutes, then transfer the muffins to a wire rack to cool completely.
Notes
If you don’t have buttermilk, you can make your own. Add 1 tablespoon of lemon juice to 1 cup of milk. Stir, and let sit for 5 minutes before using.
When adding the dry ingredients and apples to the wet ingredients, use a wooden spoon or spatula to mix everything together. Stirring or folding the mixture by hand will prevent overmixing the batter. Overmixing may lead to muffins that are tough.
Use a large scoop to fill the muffin cups with batter. This will keep the muffins similar in size and ensure even baking time.
After cooling the muffins for 10 minutes, remove them to a wire rack to finish cooling. Leaving the muffins in the pan to finish cooling will make the bottoms of the muffins soggy.