Why do my chocolate pots de creme become solid in the fridge?

I have just made the lemon pots de creme. The mixture was delicious but they all sank to half their height when I took them out of the oven. Cooling now but don't look very appetizing for tomorrow's dinner guests! What did I do wrong?

Thanks for all the info.. I had salt in both my versions, loved the flavour enhancement it created.. I'm not using any sugar (other than what's already in the chocolate) which is probably another reason why it's so thick.. will be making version #3 next weekend - third time lucky? :) At the very least, hopefully it'll be a step in the right direction..

Setting aside the taste difference, water will work just fine.

As you're working out your recipe, remember the temperature should be consistent from batch to batch. We've discussed the effects of reducing the amount of chocolate (by weight), the percentage of chocolate (e.g. 60% vs. 70% vs. milk) and the number of yolks. Increasing the amount of sugar will also thin a custard (molecularly sugar acts as a diluent). And a pinch of salt will help flavors come alive (yes in a sweet dish) as well as soften the results.

Yes, it's all about proportions. We're all saying the same thing here, just from different perspectives. Do you want a chocolate custard or a CHOCOLATE custard? If the latter, don't reduce the chocolate. Simply increase the cream.

Chocolate composition is a complicated subject but in the simplest terms, all chocolate is composed of varying amounts of three things: cocoa, sugar and milk solids. The difference between a 70% bar and a 60% bar is less cocoa and more sugar. Milk chocolate contains substantially less cocoa, maybe a third or even less than a 70% configuration. In a dish that contains cocoa, sugar and cream, it really doesn't matter where the ingredients come from, sugar in the chocolate or sugar added separately for example, just as long at the proportions are correct. My advice is to find a brand you like the flavor of and work from there.

Julia Child's recipe for Pot du Crème au Chocolat calls for medium cream. Heavy cream contains a higher concentration of butterfat. And remember there's fat in the yolks and the chocolate. So, back to proportions and that it doesn't matter where an ingredient, in this case fat, comes from. Heavy cream + milk = lighter cream.

However, cream + water is not the same thing. This is easily demonstrated. Dilute some cream with increasing amounts of water and see if you ever get to something resembling milk.

I also think the proportion of chocolate is the major factor here. Petitbleu's example of ganache is a good illustration - the higher chocolate/cream ratio, the more it solidifies in the fridge (also much harder than when simply cooled to room temp.)

I guess what I'm still most confused about is that it came out of the oven fine, was perfect when it cooled, and then it completely solidified in the fridge.. wondering if there's any one factor that would specifically cause that.. seems like 2 people plus Cooks Illustrated think it's the chocolate (either percentage or amount), although one strongly disagrees..

Here is a link to my recipe. I use half whole milk, half cream, and milk chocolate which tends to keep it nicely tender. http://thesolitarycook.wordpress.com/2012/03/04/sunday-dessert-spice-of-life-pot-de-creme/

wondering why you don't work off of one recipe. Here are just two, though it sounds like you have many

http://www.epicurious.com/recipes/food/views/Chocolate-Pots-de-Creme-104100

http://www.marthastewart.com/354091/chocolate-pots-de-creme

You're playing with a number of factors here: If you reduce the eggs, the dish will lose richness and if you reduce the chocolate… you don't really want to do that do you? All other factors being the same, substituting 60% for 70% is doing just that. Let me tell you from experience, you're nowhere near the top end of how much chocolate you can incorporate.

Heat: You don't say if you're using the traditional oven method or cooking over a burner. An oven introduces an uncertainty factor (several of them actually) making consistent results difficult. You'll have the better control on a cooktop but, either way, use a digital thermometer to determine a safe and consistent finishing temperature. Undercooking is dangerous and unnecessary.

You can thin the custard's texture by adding more liquid. When I was developing my recipe, I found using all heavy cream was a little too much with respect to the amount of chocolate (another variable you can experiment with).

I agree with clementine--sounds like the ratio of chocolate is too high for the results you want (although just the image in my head of the spoon standing up in the chocolate is making my mouth water).
I always think of ganache as a great example of what chocolate can do to desserts like this--more chocolate and the consistency of the ganache is thicker and fudgier, less and it's runnier, more spreadable.
I would try decreasing the amount of chocolate and go from there. The good news is...it's chocolate! How bad can the results be?

It sounds like you're following a recipe, but the amount of chocolate in the recipe may be too high. I've heard about 4 oz. chocolate per quart liquid. The percentage of the chocolate may affect it as well. I've had this problem before. They are supposed to set up under refrigeration and be served cool, not cold, as the flavors are better when not frigid.