Pre-heat the oven to 375º F and line muffin tins with cupcake liners (or grease the muffin cups if not using liners).
In a large bowl, cream (or mix) the butter and sugar together, 'til light and fluffy.
Then mix in the egg, buttermilk, and vanilla extract.
In a separate bowl, whisk together the flour, xanthan gum, baking powder, and salt.
Add the dry ingredients into the muffin batter, mixing for a couple minutes.
Gently fold the blueberries into the muffin batter by hand.
Pour or scoop the batter into the prepared muffin cups until each one is about 3/4 of the way full.
Bake at 375º F for 25-30 minutes, or 'til the tops are golden and a toothpick inserted into the center of a muffin comes out clean.
Once the muffins have baked, let them cool in the pan for about 10 minutes; then put them on a wire rack to cool.
Serve them with butter, jam, or whatever you like.