Combine starter, 2 cups flour and milk in a large bowl. Stir to combine, cover with plastic wrap, and leave out for at least 8 hours, or overnight.
After the overnight rest, add the remaining flour, sugar, salt, and baking soda and mix well.
Turn onto a lightly floured board and knead 4-5 minutes until dough is smooth and elastic.
Roll out to 1/2" and cut into rounds using a 3" cookie or biscuit cutter. You can re-roll the scraps but you need to let the dough rest at least 10 minutes before cutting more muffins from them.
Place muffins on a piece of parchment or cookie sheet dusted with cornmeal. Let rest for at least 60 minutes (longer is better).
Spray griddle or skillet lightly with oil. Heat to medium-low and cook muffins for about 6-8 minutes on each side or until browned on top and bottom and cooked through.
They will keep for about 3 months in the freezer. To reheat, microwave 45 seconds to 1 minute. Split and toast as usual.Always make sure you use a fork to stab all the way around the muffins to separate the halves instead of using a knife. Also, the longer you can allow the cut muffins to rest, the larger the nooks and crannies will be.If the center is still slightly underdone you can always stick them in the oven at 350°F for about 5-10 minutes until they are done.Use a digital kitchen thermometer to make sure the muffins have cooked through. They should be between 195° and 200°F