Stir together 240 grams bread flour, 85 grams whole wheat flour, 7 grams instant yeast, and 9 grams table salt in the bowl of a stand mixer or a large bowl.
In a smaller bowl or large measuring cup, stir together 227 grams milk, 57 grams plain Greek yogurt, and 12 grams olive oil.
Using a wooden spoon, dough whisk, or simply by holding the dough hook from your stand mixer, stir the milk mixture into the flour mixture until most of the liquid is incorporated.
Fit the dough hook and the bowl onto the stand mixer and turn the mixer on to low speed. Mix until the dough forms a smooth ball and no longer sticks to the bowl (about 5-7 minutes).
Knead the dough a few times by hand until it forms a ball. Place the dough into a lightly greased large bowl and cover with a lid, plate, damp towel, or plastic wrap. Let the dough rise until it's doubled (about 1-2 hours depending on the temperature of your kitchen and the milk you added to the dough).
Punch the dough down and divide it into 6 equal pieces (the most accurate way to do this is to weigh the dough and divide the weight by 6, then weigh out each piece). Keep the dough balls covered with a clean, damp kitchen towel to prevent it from drying out.
Heat a large skillet (preferably cast iron) over medium heat. Once it's hot, pour a small amount of oil (around 1 teaspoon) into the skillet, and carefully use a clean paper towel to wipe it around the skillet and remove the excess.
Working with one piece of dough at a time, roll the dough into a circle (it should be roughly 6 inches). Use a fork to poke the dough all over to prevent any large bubbles from forming while cooking. Use a small spray bottle (make sure it's only ever been used for food-safe ingredients) or a silicone brush to brush a very thin layer of water over the top of the dough.
Place the dough, wet side down, into the skillet and cover the skillet with a lid. Cook until the bottom of the dough is well-browned in spots (about 4-5 minutes), then flip and cook, uncovered, until the other side of the dough is browned, about 2-3 minutes. If you find the flatbreads are getting too dark, reduce the temperature (I find that I'm frequently adjusting between heat levels 4 and 5).
Transfer the flatbread to a wire rack and immediately brush one side of it with melted butter and sprinkle with a small pinch of kosher salt. Keep the flatbreads warm in a low (200°F) oven, if you plan to serve them right away.
Repeat with the remaining dough pieces, rolling out and prepping the next piece of dough while the previous one cooks, until all of the dough is cooked. Serve hot. If needed, reheat in a toaster oven or microwave before serving.