In a large bowl, whisk together sourdough discard, milk, and yogurt until well-combined.
Add the flours, baking powder, salt, and sugar, and stir to combine. The dough should still be somewhat sticky and tacky but not dry.
Cover the bowl with a damp cloth and let the dough rise in a warm place for 2-3 hours. The dough will be ready when you can gently dent the dough with your finger and it springs back.
On a lightly floured surface, knead dough until smooth, about 1-2 minutes.
Divide the dough into 8 pieces, using a pastry cutter or sharp knife. Using a rolling pin, roll each piece of dough into about ¼" thickness. Add a bit more flour if dough is still too sticky.
Preheat a cast-iron or stainless steel skillet over medium-high heat.
Using a pastry brush or spoon, add a small amount of melted butter into the hot skillet. Add one rolled out piece of dough to the skillet and cook for about 1-2 minutes, until the dough starts to bubble up. Brush the top of dough with butter before flipping to cook the other side for an additional minute.
Transfer to a large plate and cover with a cloth to keep warm. If serving later, transfer naan to a baking sheet and cool completely before storing in refrigerator or freezer. Repeat with remaining pieces of dough.
Garnish with cilantro before serving (if using).