Preheat the oven to 350°F (180°C). Line 10 muffin cups with liners or lightly grease them.
Whisk brown rice flour, corn starch, potato starch, cocoa powder, baking powder, salt, and brown sugar in a large bowl.
Add the oil, vanilla extract, and vegetable milk and stir until just combined. Add half of the chocolate, and leave the rest to end up the muffins.
Pour the mixture into the prepared pans. Fill the muffins 3/4 full, dividing as evenly as possible.
Bake for 25 minutes or until cooked. Keep in mind that all ovens are different, to know if it’s ready insert a toothpick into the center of the muffin, it should come out clean, with no streaks of batter.
Five minutes before they are ready, add the rest of the chopped chocolate on top of the muffins.
Let cool before removing.
Notes
Store the muffins in an air-tight container at room temperature for up to 3 days or in the refrigerator for up to 5 days.