In a large bowl, combine the bananas, milk and vinegar. Let stand for 5 minutes.
In a bowl or plastic bag, combine the sorghum flour, soy flour, tapioca starch, brown sugar, baking powder, baking soda, xanthan gum and salt. Mix together well and set aside.
Add the egg yolk and oil to the banana mixture and, using an electric mixer, beat it until all is combined. Add the dry ingredients and mix just until combined.
Lightly grease a 5 3/4 - by 3 1/4-inch, 14 by 8 cm mini loaf pan.
Spoon the batter into the prepared pan. Let it stand for 30 minutes. Meanwhile, preheat the oven to 350°F (180°C).
Bake for 20 minutes. Check to see if the loaf is getting too dark and tent with foil if necessary.
Bake for an additional 15 to 20 minutes or until internal temperature of loaf registers 200°F (100°C) on an instant-read thermometer.
Let the banana bread cool in pan on a rack for 5 minutes. Remove from pan and let cool completely on rack.