First, in a large bowl or a bowl of a stand mixer fitted with a dough hook, measure out the active sourdough starter, water, and sugar. Mix this by hand or spoon to create a slurry.
Next add the milk, butter, and flour. Mix this together by hand or dough hook until a shaggy dough forms. Scrape the sides of the bowl as needed. Cover with a tea towel and let sit for 20 minutes. This is the autolyze.
After the 20 minutes add the kosher salt and knead by hand or dough hook for 6 minutes. If in a stand mixer bowl transfer to a lightly greased bowl.
Let the dough rest covered by a tea towel for 4-8 hours. This is the bulk fermentation. You want the dough to be risen, soft and puffy. If you have a cool kitchen, you could do this on your counter overnight.
Place the dough, covered, in the refrigerator for 2 hours. This helps the dough to firm up and be easier to work with as it is a stickier dough. The first rise is complete.
Take the dough from the refrigerator and place it on a clean work surface. Using a rolling pin or your hands, roll out or pat the dough to about ½ inch thick.
Using a 3 in. biscuit cutter cut out your English muffins and place them on a baking sheet generously dusted with cornmeal. You can lay parchment paper down first if you prefer.
Make sure you don’t skimp in this area otherwise the muffins will stick to the pan and be hard to get off when proofed and ready to cook.
You can also have a bowl of cornmeal and “dip” the bottoms of the English muffins in it before setting on the cornmeal dusted baking sheet.
You will have scraps after you get the first round made. You can easily work this into a ball and roll it out again. Make sure to let the rerolled dough rest for 5-10 to let the gluten relax. Otherwise, your English muffins with shrink after being cut.
Sprinkle the tops with some corn meal.
Cover with a damp tea towel and let proof for their second rise, in a warm place. This can take 3-5 hours depending on the temperature. You want them to be risen and puffy.
Preheat a cast iron skillet over a medium-low heat. Place 4 English muffins in the pan and cook for 6-7 minutes on each side or 12-14 minutes total. You want them to be golden brown and cooked thoroughly.
Remove from the pan and place on a wire rack to cool.
Repeat with the remaining English muffins.
When you are done cooking and cooling all the English muffins you can either store them in an airtight bag or airtight container at room temperature for about a week or freeze them for longer storage and future use.