Sourdough English Muffins

Name: Sourdough English Muffins
Ingredients: 250 grams (8.8 ounces) bread flour (i used the french farine bise t80, which is about halfway between white and whole wheat); 70 grams (2.5 ounces) ripe natural starter (fed 6 to 8 hours before; the idea is to use it when it's almost at its peak)*; 1/2 teaspoon sea salt; 1/2 teaspoon instant yeast (levure boulangère déshydratée in french); 1/2 tablespoon agave syrup (substitute 1/2 tablespoon honey or sugar); 15 grams (1 tablespoon) butter, softened; 150 ml (1/2 cup plus 2 tablespoons) milk, plus a little more if necessary (you can also use buttermilk); Cornmeal, for sprinkling