Preheat oven to 180 degrees C. Grease two 12 cup muffin trays or line with patty cups.
In a medium bowl, stir together the rice flour, millet flour, cocoa, salt, bicarb soda, baking powder and xanthan gum. In a separate large bowl, beat the eggs, sugar, sour cream, milk and vanilla. Stir in the dry ingredients until smooth. Spoon the batter into the prepared cups, dividing evenly.
Bake in the preheated oven until the tops spring back when lightly pressed, 20 to 25 minutes. Cool in the tray on top of a wire rack. When cool, arrange the cupcakes on a serving plate.
Some people are extremely sensitive to gluten. Please check the labels on cocoa, baking powder and vanilla to be sure they are gluten free before serving to someone with an intolerance.