Duomei Chocolate flavour concentrate.
 
Start low with Duomei flavours, maybe around 1% for standalone  and increase it if it was not enough.
As an addditive start lower than 1%.
 
Do you like JF Omega Chocolate? if yes try this and see which one will beat the heat! you would be excited to see how profound and ymmy it is, at the same time not too strong to overwrite other flavours too.