Place the ice cube molds in the cooler, making sure there is space between the molds and the walls of the cooler. Fill the cooler with hot water so that the molds and the space between them and the cooler are both completely filled. Do not add so much water that the molds float. Do not place a lid on the cooler.
Place the cooler in the freezer (without the lid!) and freeze until solid — 14 hours to 2 days.
Remove the cooler from the freezer. If frost has formed on top of the ice, pour on enough fresh water to cover, slosh, and immediately dump the excess. Then, park the cooler on the counter until you’re able to jiggle the molds free of the surrounding ice — about 1 hour.
When the molds are free of the cooler, push out the cubes. To remove any foggy bottoms, simply place on a wooden cutting board (or even the cooler lid) with the foggy bottoms facing upwards. Place an aluminum sheet pan on top of the cubes and move it around gently until the foggy parts melt away, a process that will happen a lot quicker than you’d think.
Dry the cubes with a clean, lint-free towel and transfer to a zip top freezer bag. Return to the freezer until ready to use. (Only use in drinks requiring one nearly perfect cube.)