Make the Dough and Let Rise:

1. In a large bowl, mix bread flour, whole grain emmer (or whole wheat) flour, and kosher salt together until well combined. Add kefir milk, sourdough starter, honey, and egg white, stirring with a wooden spoon or flexible spatula until smooth, about 5 minutes.
2. Cover with plastic and set aside until spongy, light, and more than doubled, 4-6 hours at 70°F. (The timing can be flexible depending on your schedule.)

For the Second Rise:

1. Thickly cover a rimmed aluminum baking sheet with an even layer of cornmeal. With a large spoon, dollop out twelve 2 2/3-ounce (75g) portions of dough; it’s perfectly fine to do this by eye. If you’d like, pinch the irregular blobs here and there to tidy their shape.
2. Sprinkle with additional cornmeal, cover with plastic, and refrigerate at least 12 and up to 42 hours.
3. Instead of forming blogs with a spoon, I shaped the very sticky dough into balls using wet fingers, and placed them on a cornmeal-dusted rimmed baking sheet. I ended up with 11 muffins.

To Griddle and Serve:

1. Preheat an electric griddle to 325°F or warm a 12-inch cast iron skillet or griddle over medium-low heat. When sizzling-hot, add half the butter and melt; griddle muffins until their bottoms are golden brown, about 8 minutes. Flip with a square-end spatula and griddle as before.
2. Transfer to a wire rack until cool enough to handle, then split the muffins by working your thumbs around the edges to pull them open a little at a time.
3. Toast before serving and store leftovers in an airtight container up to 1 week at room temperature (or 1 month in the fridge).