Preheat oven to 180°C.  Lightly butter or grease a 12-cup muffin pan or line with paper liners. 

Stir together flour, sugar, baking powder, xanthan gum and salt in a medium bowl until blended. 

Whisk together butter, milk, eggs, lemon zest and vanilla in a large bowl until combined. Stir in flour mixture until partially moistened. Fold in blueberries until evenly mixed. Batter will be thick, do not over stir. 

Divide batter evenly between muffin cups, filling each about three-fourths full. Bake 10 minutes. 

Rotate pan and continue to bake about 10 minutes longer or until muffins are golden brown and toothpick inserted in centre comes out clean. Cool in pan 3 minutes. Serve warm or remove to wire rack to cool completely.