Mix the starter ingredients in a medium bowl (because it bubbles up). Cover and keep at room temperature for 12-36 hours. The longer it sits, the better the muffins will taste.
Using a stand mixer, combine the milk, yeast and the starter. Beat well so the starter is well incorporated into the milk.
Add the sugar, cinnamon, raisins, butter and salt. Add the flour, beginning with the lowest amount called for. Switch to the dough hook and knead for 8 minutes, adding additional flour so that the dough is soft yet not sticky. It should feel slightly tacky but not stick to your fingers nor the bowl.
Place dough in a lightly oiled large bowl, cover and let rise in the fridge overnight or up to 3 days.
Place dough on a lightly floured surface and shape into a round disk. With a large knife, cut 12 equal portions. Roll each portion into a smooth round ball.
Sprinkle some of the cornmeal on a large baking sheet. Place the balls about 2-inches/5 cm apart. Sprinkle the tops of the balls with cornmeal as well. Let rise until puffy--for cool dough this will take about 1 1/2-2 hours.
Heat a large skillet or griddle over medium heat. Grease the skillet or griddle with the butter (I use a tissue to distribute the butter then use the buttery tissue to grease the griddle for the next batches).
Working in batches (probably about 3-4 muffins depending on the size of your pan/griddle. Don't crowd the pan) cook the muffins for 5-6 minutes on one side until they're golden brown.
Flip the muffins and cook for 5-6 minutes on the other side, gently pressing the muffins down with a wide spatula to flatten slightly.
For extra insurance that the muffins are fully cooked, you can put them in a pre-heated 350F/180C oven for 5-8 minutes.