Preheat your oven to 375°F. Grease or line a standard 12-cup muffin tin with muffin liners, and set it aside.
In a large mixing bowl, place the flour blend, xanthan gum, baking powder, baking soda, salt, and cocoa powder, and whisk to combine well.
In a separate medium-size, heat-safe bowl, place the chopped chocolate and place over a small pot of simmering water, making sure the water in the pot doesn’t touch the bowl. Melt the chocolate, stirring occasionally, until smooth.
Remove the melted chocolate from the heat, add the oil and whisk until smooth.
To the chocolate mixture, add the granulated sugar and the brown sugar, and whisk to combine. Break up any lumps in the brown sugar.
Crack the egg open into the chocolate mixture, and whisk the egg into the mixture until smooth.
Next, add the buttermilk and vanilla, and whisk again until smooth. The mixture will be thin.
Create a well in the center of the dry ingredients, add the entire chocolate mixture, and mix until just combined. The batter will be thick but should be relatively shiny and soft.
Mix in the chocolate chips. You can reserving a few chips to scatter on top of the muffin batter in the wells of the muffin tin, if you prefer.
Divide the raw muffin batter among the wells of the muffin tin. Each well should be filled or nearly filled with batter.
Top with any reserved chocolate chips, and press them down gently so they aren't just sitting on top of the batter.
Place the tin in the center of the 375°F preheated oven and bake for 5 minutes.
Reduce the oven temperature to 350°F, and continue to bake for about 14 minutes, or until the muffin tops are firm to the touch and a toothpick inserted in the center of the middle muffin comes out with no more than a few moist crumbs attached. Do not overbake or the muffins may burn.
Remove from the oven and allow to cool in the tin for 5 minutes before transferring to a wire rack to cool completely.
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Notes
Nutrition information is an estimate per muffin made using online calculators. It is provided as a courtesy and it should not be relied upon.