Whisk egg yolks and gentle sweet together in small saucepan until thoroughly combined & lightened in color, about 2 minutes.
Add cashew milk and whisk well again.
Cook over low/medium heat stirring with wooden spoon constantly until mixture reaches temperature of 170°, about 5 minutes. It will coat back of spoon and it may appear to be curdled, but don't worry..it'll smooth out in the next step.
Remove from heat and place in blender.
Add vanilla, glucomannan, cream of tartar, & molasses. Blend on low for 1 minute, venting blender as needed.
Scrape into mixing bowl (or back into pan if using a hand mixer).
Let cool until mixture reaches about 100°. You may place in a sink of cold water to cool faster, if desired.
Add butter 1 piece at time, waiting until each piece of butter is COMPLETELY mixed in before adding another. Mix on high speed using whisk attachment until smooth. This may take several minutes. If mixture won't come together or appears curdled after mixing for several minutes, it probable means your mixture and/or your butter was too cold. If this happens, warm mixture very gently over low heat or microwave for several seconds and whip for a few more minutes.