Preheat the oven to 400°F.
Grease the cups of a 24-cup mini muffin tin with butter or nonstick cooking spray. Place paper liners in the greased cups.
In a medium bowl, mix together flour, xanthan gum, baking powder, and sugar. Add the melted butter and stir to form a crumbled mixture. Set aside 1/2 cup of the mixture for the topping.
In a separate bowl, mix together the half-and-half, eggs, and vanilla.
Add to the crumbled mixture, stirring just until moist to make a batter.
Fold blueberries into the batter.
Scoop the batter into the prepared muffin cups. A cookie scoop works well to transfer the batter. Sprinkle the tops of the muffins with the reserved crumble mixture.
Bake for 12-15 minutes, until the muffins spring back when lightly pressed and a toothpick comes out clean. The muffins should be slightly golden brown on top.
Let the muffins cool in the tin for 5 minutes, then remove and serve.