Check the in-game gacha or featured banner for Shadow Milk Cookie
Use Cookie cutters, crystals, or other summon currency on the banner where Shadow Milk Cookie is available
Wait for a rerun banner if Shadow Milk Cookie is not currently available
Check event rewards or limited-time missions for possible Shadow Milk Cookie acquisition
Use mileage, pity, or exchange systems if the game offers them for this Cookie
Check the shop or special packs for direct purchase options if available