Grease and/or line 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, optional butter emulsion, melted vegan butter, and oil to the bowl, and whisk vigorously to combine very 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.
Set the batter aside for at least 20 minutes, ideally 30 minutes. It will thicken as it stands.
While the muffin batter rests, preheat your oven to 400°F.
Use a silicone spatula to turn the thickened muffin batter over a few times. Add almost all of the mix-in pieces, and mix until evenly distributed throughout. Reserve a few chips or other mix-ins.
Fill the prepared wells of the muffin tin all the way with batter. Pile the batter toward the center of each well if at all possible. Top evenly with any remaining mix-in pieces and press in gently to help them stick to the batter. Sprinkle the top of the batter in each well lightly with the optional coarse sugar.
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
Flour blend information.My favorite vegan-friendly gluten free flour blend is Better Batter's original blend gluten free flour. Bob's Red Mill 1-to-1 Gluten Free Baking Flour is also dairy free and works well. I recommend adding an additional 1/4 teaspoon xanthan gum to your dry ingredients or the muffins will look good, but will be crumbly. To make your own blend using my “mock” Better Batter recipe, please see the all purpose gluten free flour blends page.About the muffin tin If you don't have a 6-well muffin tin, use a standard 12-well muffin tin and fill every other well. 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) muffins. Just double all the ingredients by changing the yield to 12 or clicking the [2x] button.