"...it works well except when using cocoa butter..."
Well, actually a hot lye solution will melt cocoa butter just fine ... if you are not using a large amount.
The real limitation of the "heat transfer" method is that the hot lye is not able to melt a large amount of solid fats, especially fats with a higher melting point -- lard, tallow, the butters, palm. The lye solution doesn't contain enough thermal energy to do the job.
If you are making soap that is largely liquid fats with small amounts of solid fats, put the hot lye solution with just the solid fats first. When all is fully melted, add the liquid fats. If the lye solution can't fully melt the solid fats, you still have the option of warming the liquid fats before adding them, so they can finish the job. If the lye solution is hot enough to do the job, then the room temp liquid fats will cool the mixture somewhat so the batter isn't overly hot.