The night before, add sourdough starter, water, honey, and whole wheat flour to a medium mixing bowl. Stir well. If the dough seems too wet, you can add more flour (up to 1/4 cup). Stir well to combine. You don't want the dough too dry though, so be careful not to add too much additional flour.
Cover loosely with a tea towel or plate and let it sit overnight (8-12 hours).
The next morning (or whenever you're ready to make your muffins), add the salt and baking soda to the dough and mix well. Mix/knead until well combined. You can do this by hand, but I usually put mine in a stand mixer with the dough hook, and add the salt and baking soda, and let it knead the dough for a couple of minutes. If the dough is still wet, I add up to 1/4 cup of flour to the mixing bowl and knead it in.
Preheat a cast iron skillet or griddle on low to medium heat. Add about 1 tablespoon of butter or coconut oil to grease the pan, if desired.
Drop the muffin dough from a spoon (or 1/4 cup measuring cup) onto the skillet, or use your hands to shape the dough. I grease my hands with a little bit of coconut oil, remove about 1/4 cup of dough, and form it into a loose ball shape before placing it into the skillet.
Cook the muffins for about 3-5 minutes, and check to see if the first side is browned. If it is, flip the muffins over and then cook the other side until browned.
If it seems like they are browning too quickly and the center isn't getting done, turn the heat down and cook more slowly. You can also transfer them to a baking sheet, and bake them in a 350 degree oven for a few minutes to finish the cooking process.
Transfer muffins to a cooling rack for at least five minutes, and then slice with a serrated knife. Let cool completely before storing in an airtight container.