Line four 12-hole mini-muffin tins with 40 paper cases. Sift the flours, bicarbonate of soda, baking powder and xanthan gum together into a large bowl, then stir in the sugar.
Mix together the butter, egg and buttermilk in a separate bowl. Gently combine the dry and wet ingredients, and lightly fold the chocolate drops into the mixture, stirring well.
Spoon the mixture into the paper cases and place in a preheated oven, 200°C (400°F), Gas Mark 6, for 15 minutes until golden and risen. Remove the cakes from the oven and transfer to a wire rack to cool.
Melt the rest of the milk chocolate and drizzle over the cooled muffins before serving.