Parchment-line a large tray and clear a level refrigerator shelf.
Whisk quick oats, flax, cocoa and salt in a large bowl; add optional coconut.
Warm peanut butter, maple syrup, coconut oil and milk over low heat for 2 to 3 minutes, whisking until smooth and glossy without boiling.
Remove from heat, whisk in vanilla and stand 1 minute.
Fold binder into the oat mixture until every oat is coated and no dry cocoa remains.
Rest 5 minutes. If it smears, add quick oats 1 tablespoon at a time; if it crumbles, add milk 1 teaspoon at a time.
Scoop 18 portions with a 1 1/2-tablespoon scoop onto the lined tray.
Press each mound into a uniform half-inch disk and smooth large edge cracks.
Refrigerate 45 to 60 minutes until cookies lift cleanly, then store covered and cold.
