In a large bowl, whisk together bread flour, warm water, sourdough starter, dry milk powder, melted butter, sugar, salt, and instant yeast until smooth and well combined.
Cover the bowl with plastic wrap and let the batter sit at room temperature until it doubles in size, roughly 1 hour.
Heat your griddle over medium heat until evenly warm. Lightly grease the inside of eight 3 1/2-inch ring molds with cooking spray and place them on the griddle.
Without stirring the batter, scoop it using a 1/3 cup measuring cup and pour evenly into each ring mold.
Place a baking sheet over the molds to trap heat. Reduce the heat to low and cook the muffins for 5 minutes without disturbing them.
Remove the baking sheet. Carefully use a spatula to flip each muffin while holding the ring in place with tongs.
Cover the rings again with the baking sheet and cook for an additional 5 minutes.
Remove the baking sheet and tongs to lift off the molds. Cool the muffins on a wire rack for 10 minutes before splitting with a fork or by hand.
Toast the muffins before serving if desired, and enjoy with your favorite toppings.