1 drop Capella Golden Butter per 10ml. If you use Butter and don't let this age for 2-3 weeks, you will have a very buttery custard (which may be okay for some people). Just a heads up, Capella renamed "Butter" to "Golden Butter" so everything is pretty much the same as before.
Feel free to sub marshmallow with whatever brand you have. I've made this with cap and tfa before and didn't notice too much of a difference in the overall flavor.
If you prefer your custards to be a little less sweet or plan to age this for an extended period of time, just change the Vanilla Custard and French Vanilla Percentages at a 4:1 ratio. (ex. 8% VC & 2% FV or 6% VC & 1.5 FV)
Give this one at least a week to age but your sweet spot will be around week 3
Hey ELR, here's an alternate that I enjoy just as much as what's posted here.