Preheat your oven to 400°F. Grease 6 standard-sized muffin wells and set the pan aside. (See Recipe Notes about muffin tin.)
In a large bowl, place the granulated sugar and light brown sugar and mix them together, breaking up the lumps in the brown sugar.
Add the milk, vinegar, vanilla, melted vegan butter, and oil to the bowl, and whisk to combine well. The mixture will be thin.
Scatter the dry ingredients, first the flour blend and xanthan gum, then the tapioca starch, baking powder, baking soda, and salt, on top of the wet ingredients. Whisk to combine until very smooth. The batter will be sticky and runny.
Add just enough of the plain muffin batter to each prepared well of the muffin tin to cover the bottom (about 1.5 tablespoons of batter in each well).
To the remaining batter in the mixing bowl, add the mix-in pieces, and mix until evenly distributed throughout. Fill the wells the rest of the way full. Wipe up any spilled batter with a towel, as needed.
Place the baking tin in the top third of the preheated oven and bake for 10 minutes. Reduce the oven temperature to 350°F and continue baking until a tester comes out mostly clean and the muffins are nicely domed (about another 12 minutes).
Remove the muffins from the oven and allow them to cool in the tin for 5 minutes before transferring to a wire rack to cool completely.
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Notes
About the muffin tin If you don't have a 6-well muffin tin, use a standard 12-well muffin tin. To ensure even baking, fill the 6 empty wells about 2/3 of the way with lukewarm water.To make 12 muffins This recipe doubles really easily to make a full dozen (12) vegan gluten free muffins. Just double all the ingredients.To make 3 muffins If you'd like to halve the recipe and make just 3 muffins, cut all the ingredients in half and fill the empty wells 2/3 of the way with lukewarm water, as above.