Preheat the oven to 350 degrees.
Place baking cups in a dark muffin pan.
In a bowl, mix together gluten-free flour, xanthan gum, salt, and baking powder.
In a separate bowl mix together vegetable oil, sour cream, milk, and egg. Stir in the sugar.
Fold in the dry ingredients and mix until well combined.
Stir in chocolate chips.
Pour the batter into the baking cups. About 1/3 cup of batter in each cup.
Mix together the brown sugar and sugar for the topping in a small bowl.
Sprinkle the top of each muffin with the sugar mixture using your fingers.
Bake for 25 minutes until an inserted toothpick comes out clean.
Remove muffins from the pan and cool on a wire rack.
Serve.