How can I make a stable olive oil and vanilla ganache?

Disclaimer: I've never tried to make a ganache with olive oil, so take that into consideration as you read the following.

First, I have to assume that you're trying to use olive oil to replace the cream that's normally used in making ganache, perhaps to make a non-dairy ganache. If that's not right, please clarify your question.

Ganache is essentially an emulsion of cocoa butter (usually from chocolate) and cream, plus vanilla and/or other flavorings, that is allowed to cool to the point where the cocoa butter solidifies. It's the solidified cocoa butter that gives ganache it's firm texture. The greater the ratio of cocoa butter to cream, the firmer the ganache will be. If you want to replace the cream, you'll need to make sure that you replace all the components that have an effect on the emulsion. Harold McGee's On Food and Cooking explains the structure of ganache:

The continuous phase of this mixture, the portion that permeates it, is a syrup made from the cream's water and the chocolate's sugar. Suspended in the syrup are the milk fat globules from the cream, and cocoa butter droplets and solid cocoa particles from the chocolate.

He doesn't say it, but I'd guess that the milk proteins from the cream might act as emulsifiers to help stabilize the emulsion, much as they do with butter.

The melting point of olive oil is obviously different from that of butterfat, but since you heat the cream in order to make ganache anyway, olive oil might still work. Given what McGee says, you'll need to make sure that you add an appropriate amount of water so that there's enough syrup to suspend the butterfat, cocoa butter, and solids. How much is an appropriate amount? Cream is usually 20-40% butterfat, so starting with a 1 part oil for 2 parts water seems reasonable. You might also need to add something that'll act as an emulsifier, like lecithin.

Pierre Herme lists a recipe for this in his book Macarons. He just makes a white chocolate ganache with cream, and then blends in olive oil with a hand blender. The amounts suggest he is replacing much of the cream with oil (350g white chocolate, 150g cream, 225g olive oil). I've heard the flavor is amazing, but haven't made them myself.

I have tried this olive oil ganache, it works well and it is very stable.

Recipe:

Ingredients: 350g white chocolate (I used the Valrhona), 150g cream, 225g olive oil, a vanilla bean.

Preparation: Heat the cream with the vanilla bean slowly (divide the vanilla bean in two, and take all the vanilla inside it). As it starts boiling, take it off the heat. In a double boiler, melt chocolate and stir. Increase the temperature of the olive oil to at least 35ºC. If the vanilla cream has cooled down to 55ºC, take 1/5 of the cream, and stir it in the melted chocolate until you have a homogeneous mixture, then take another fifth and do the same. Repeat the process three more times, so you'll have a stable ganache. Then, start with the olive oil, repeating the process you have done with the cream (so by fifths, adding a bit of oil to the cream/chocolate mixture). In the end, you'll have a beautiful rich and flavoured ganache.

I tried it and it really worked. I also tried it with dark and milk chocolate, and it worked better, but the original recipe is from Pierre Hermes.

If you want a more solid structure to your olive oil ganache, you should probably try the Ferran Adria emulsification of olive oil first. It will have a more stable structure when mixed with the Pierre Hermés recipe.

• For each 100g of olive oil, add 6 Gr of Glice (these are flakes of Glycerine)
• Mix everything and warm in a small pan
  • Don't over heat the olive oil
  • Slowly cook the mixture and remove from heat as the flakes dissolve
• Keep mixing until it starts to solidify

This can be done some days earlier and it works as a "solid olive oil butter". You don't need to put all the olive oil mentioned in the recipe, scale it back but be sure to blend the olive oil slowly, and taste it until it has enough of the olive oil taste.

I would omit the vanilla, as both it and olive oil are singular and unique flavors; but that is your choice.