Milk soaps replaces all the water in a cold process recipe with milk. This can help make a soap extra smooth with a creamier lather. However, using milk in a cold process soap recipe makes it trickier to add lye.
Lye is necessary to make soap its soapiest, but when added at the wrong time with milk soap recipes can scorch the milk. This can make the milk soap smell bad. A solution to this problem is to freeze the milk ahead of time and add lye flakes slowly while stirring. This can keep the temperature low enough to prevent scorching. For a full tutorial on cold process milk soap, click here:: How to Add Lye to Milk