Preheat your oven to 350°F. Grease or line the wells of a standard 12-cup muffin tin and set it aside.
If making the (optional) crumble.
If you’d like to add the crumble to the tops of your muffins, make the crumble now. Place all the (optional) crumble ingredients in a small bowl and mix to combine fully.
Place the crumble mixture in the refrigerator to chill while you make the muffin batter.
Make the muffin batter.
In a medium-size bowl or large measuring cup, place the eggs, buttermilk, butter, and vanilla, and mix until well-combined. Set the wet ingredients aside briefly.
In a large mixing bowl, place the flour, xanthan gum, baking powder, baking soda, salt, granulated sugar and brown sugar. Whisk to combine and break up any lumps in the brown sugar.
Create a well in the center of the dry ingredients and pour in the egg and buttermilk mixture. Stir the wet ingredients into the dry ingredients until just combined. Don’t overmix.
If adding any simple mix-ins like chocolate chips, add them to the batter now and mix just until evenly distributed throughout the batter.
Fill the prepared wells of the muffin tin with batter, and smooth the tops with wet fingers. You should get 12 muffins, but if your wells are deeper than mine you'll get less and more shallow you'll get more.
If you’re adding the crumble, remove the bowl from the refrigerator and break up the chilled crumble into irregular pieces (none too large) with the tines of a fork.
Sprinkle the crumble generously on the top of the muffin batter in the wells, and press gently to adhere the mixture to the muffin batter.
Place the pan in the center of the preheated oven and bake until the top of the center muffin springs back when pressed gently in the center, about 20 minutes.
If you’ve added the crumble, those muffins may take another minute or so to bake fully.
Remove from the oven and allow the muffins to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.
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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 1/2 teaspoon xanthan gum to the muffin dry ingredients or the muffins will look good, but will be crumbly. The crumble topping should be fine, or you can add 1/8 teaspoon extra xanthan gum to be sure it holds its shape. 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.Substitution suggestions.Dairy free: In place of butter, try vegan butter in a block like Miyoko's Creamery brand. For buttermilk, use half (3 fluid ounces) nondairy unsweetened unflavored milk and half (3 fluid ounces) plain nondairy yogurt.Egg-free: Try one "chia egg" per egg (3 in the regular recipe). "Flax eggs" would likely work better, but tend to add an unwanted flavor and color. You can also try Bob's Red Mill egg replacer or Just Egg plant-based eggs (from the refrigerated section of the grocery store).Nutrition information.Nutrition information is approximate per muffin, including chocolate chips and excluding the crumble topping.