In a large bowl, or the bowl of your stand mixer add yeast and warm water. Let sit about 5 minutes until yeast is dissolved. Then, add warm milk, butter, shortening, salt and sugar. Mix until combined.
Add 2 cups of flour to the wet mixture and begin to knead. Add remaining 2 cups of flour and continue to knead until you have a nice soft round ball. Place dough in a lightly oiled bowl and cover with a towel. Let rise for about 1 1/2 - 2 hours, or until doubled in size.
Turn dough over onto a floured pastry mat or clean surface. Gently press dough down until its about a half inch thick. Cut muffins using your muffin rings and then transfer to a clean surface sprinkled with cornmeal. Cover with towel and let rise for another hour.
Heat a large skillet to medium-high heat. Add about 1-2 tablespoons of butter to the hot skillet and then add muffins about 3-4 at a time (or as many will fit in your skillet). Let cook until bottom is nice and golden and then flip over and cook until the other side is golden. Transfer muffins to a cooling rack.