Preheat your oven to 350°F. Grease the wells of a standard 12-cup muffin tin and set it aside.
In a large bowl, place the all purpose flour, xanthan gum, cornstarch, baking soda, baking powder, salt, and sugar, and whisk to combine well.
Add the grated carrots and mix to combine, breaking up any clumps. Add the raisins or chocolate chips, and toss to coat. (See Recipe Notes)
Create a well in the center of the dry ingredients and add the coconut oil, beaten eggs, buttermilk, and vanilla, and mix to combine. The batter will be thick but soft.
Fill the prepared wells of the muffin tins about completely full with batter. With wet fingers, smooth the tops of the batter in the wells.
Sprinkle the tops of the batter in the muffin wells evenly with the optional brown sugar.
Place the tin in the center of the preheated oven and bake until the tops of the muffins spring back when pressed gently (18 to 20 minutes).
Remove the muffin tin from the oven and place on a wire rack. Allow to sit for at least 5 minutes before transferring to a wire rack to cool completely.
Video
Notes
Flour blends.My favorite gluten free flour blends are Better Batter's original blend gluten free flour and Nicole's Best multipurpose blend. Bob's Red Mill 1-to-1 Gluten Free Baking Flour should also work, but you'll need to add an additional 1/4 teaspoon xanthan gum to your dry ingredients or the muffins will look good, but will be crumbly. Cup4Cup changed its formula and doesn't seem to work as well as it has in the past, so I don't recommend it. To make your own blend using one of my “mock” recipes, please see the all purpose gluten free flour blends page.Mix-ins.You can leave out the raisins or chocolate chips entirely, or you can use whatever you prefer in their place.Use good-quality raisins, though, or they'll be too chewy in the muffins. If your raisins seem hard, soak them in some warm water for about 15 minutes first. Then drain the water from the raisins, and add them to the batter once all the other ingredients, including the wet ingredients, have been added.