I just reviewed the Nielsen Massey website and under their FAQ's they suggest that vanilla powder be used for "liquid sensitive products". The powdered nature of the vanilla would allow you to add it to melted chocolate without causing the melted chocolate to seize up. While vanilla has a unique and characteristic flavor of its own, it also helps to heighten and intensify other flavors, chocolate being one of them.
When chocolate seizes the emulsion of cocoa butter and cocoa powder has been interrupted by the introduction of a small amount of moisture. You either need to keep all liquids out of melted chocolate or add in a significant amount.
So...it only prevents it from seizing if you were going to be adding vanilla in extract form but no other liquids. It would not be something you add as a preventative measure.
Prevent the seizing through proper procedures in the first place:
- Do not allow water in the water bath to touch bottom of the bowl that you're melting chocolate in.
- The steam produced by the water is what's doing the melting so as long as wafts of steam are rising from the surface it's hot enough. The water doesn't even need to be simmering - cocoa butter starts to melt around 83-85 degrees F.
- Make sure all utensils are dry so water droplets aren't introduced
- Make sure the bowl you're using is large enough for the edges to flare out beyond the edge of the pot. If the edges just meet the edge of the pot then steam will condense on the edge and get into the chocolate causing it to seize.
If the texture changed immediately, then that sounds like seizing to me. Chocolate shouldn't seize if you add fat (oil) to it, particularly if you add it after the chocolate is fully melted. Only water will cause that.
Olive oil varies significantly in purity and quality, so cheap olive oil may actually be worse for this application than a purer, cheaper cooking oil (such as peanut or corn oil). There may have been impurities or even trace amounts of water in the bottle. Or it could be that you just tried to melt it too fast, or you accidentally got some water in there from some other place (condensation is a common source).
If you melted it directly in a pan, as in your comment to Kyra, and continued to apply heat after adding the oil, then it's also possible that it didn't really seize at all and that you just tried to melt it too fast.
If it did seize, then there is a cure - perhaps counter-intuitively, adding more oil could help you. Using roughly a tablespoon for each ounce of chocolate will help smooth out any lumps. Add and whisk it very gradually. Of course, this will only work if the oil itself is reasonably pure.
Note: In this case, I would guess that the use of low-quality chocolate isn't your problem if you successfully melted it (albeit to a thick consistency) before adding any oil. Significant impurities in the chocolate itself would lead to seizing during the melting process.
Best Answer
If you can get instant espresso powder (most big grocery stores in the US have it) that's what you want to use. NO WATER, small amounts of water will ruin chocolate, cause it to seize.
Just sprinkle some powder in while the chocolate is melting. It will blend right in.
If you can't get espresso powder, you can use instant coffee, just be sure that it's a fine powder, or powder it yourself in a spice or coffee grinder.
To be sure that the instant coffee is fine enough, you might consider sifting it. Either powdered regular instant coffee or even the instant espresso might benefit from that.