Working somewhat quickly before the mixture stiffens, drop heaping tablespoonfuls of the oat mixture onto the prepared parchment, forming rustic clusters. Aim for compact mounds so they hold together as they cool.
Let the cookies sit at room temperature until firm and set, about 30 to 45 minutes. They should look like glossy chocolate-coated clusters that hold their shape when gently picked up.
Once fully set, transfer the cookies to your white ceramic platter and serve. Store any leftovers in an airtight container at room temperature for up to 3 days, layering parchment between cookies if you stack them.
For a slightly less sweet cookie, reduce the sugar to 1 1/4 cups; the texture will still set, just a bit softer. If you like a deeper chocolate flavor, increase the cocoa powder to 1/3 cup and add an extra tablespoon of milk to keep the mixture fluid. Crunchy peanut butter works just as well and adds texture—very much in the spirit of farmhouse improvisation. You can fold in 1/4 cup of chopped roasted peanuts or shredded coconut with the oats for more chew. For a dairy-free version, use a plant-based butter and non-dairy milk; just keep the 1-minute boil to help them firm up. If your kitchen is warm or humid, slide the tray into the refrigerator for 15 minutes to help them set, then bring them back to room temperature before serving so the chocolate coating stays glossy rather than dull.
