In a large mixing bowl, combine the sourdough discard with the warm milk and yogurt. Stir with a wooden spoon to mix.
Add all-purpose flour, baking powder, and salt into the bowl. Mix with a wooden spoon just until a dough starts to form and pull away from the sides of the bowl.
Turn the dough onto a floured work surface or board. Knead gently with your hand, pushing the dough into the board, pulling up the far edge with your fingers and folding into the dough. Rotate and repeat until smooth and elastic, about 5 minutes.
Place the dough in a clean large bowl. Cover with a damp cloth and let it rise in a warm and dry area for about 2 hours, or until doubled in size.
To see whether your dough is ready, you can gently poke it with your finger. If it springs back slightly, it’s ready.
Transfer the dough onto a work surface again. Knead with your hands for 1-2 minutes until smooth.
Divide the dough into 8 portions. Using a rolling pin, roll each piece flat to about ¼-inch thickness. (If need, you can stretch it out using your fingertips like with pizza dough.)
Place a cast iron skillet on medium-high heat for 3 to 5 minutes until very hot. Brush one piece of dough with melted butter and place it onto the hot skillet.
Cook for 1 minute undisturbed or until bubbles start to form on the surface. Then flip and cook for another minute on the other side. It should have started to puff up, and don't worry if it puffs high up as it will deflate off the heat.
Transfer the cooked naan to a plate or basket. Cover with a cloth or tea towel to keep warm. Repeat the cooking with the remaining pieces. (Note: if the dough has puffed up while sitting, run the rolling pin across it a few times to flatten before adding to the skillet.)
Top with optional minced cilantro and serve warm.