heavy cream is just the cartons of creme in the baking asile that you can whip into whipcreme. hmm buttermilk buttermilk I should know the answer to that right? I think its just a certain type of milk.
sorry I know im not much help but I hate reading a post and at least not trying to answer. 
Heavy cream is double cream (about 40% fat), buttermilk is soured milk - I looked up the English-French translation and it says it's 'babeurre' in French - does that make sense?
You can make a pseudo buttermilk by adding 1 tsp of white vinegar to a cup measure and filling the rest up with milk. Works the same. Heavy cream is also called "coffee cream" and "whipping cream" here in the States. It's very thick and has a good whipping consistency.
Buttermilk History
In days gone by, nothing went to waste in the standard homestead, and this included the liquid leftover after churning butter. Combined with natural airborne bacteria, this liquid thickened and soured, taking on a pleasingly tangy flavor. The resulting buttermilk made an excellent addition to biscuits, pancakes, and baked goods.
Modern buttermilk from the dairy has acidic culture added to it and allowed to ferment under controlled conditions. As Bobwonderbuns said you can create a subsitute by adding vinegar or lemon juice to milk. You can also get dried buttermilk in a canister, which is what I use. Some recipes require buttermilk because they use baking soda rather than baking powder--one of those chemistry things. Most of the recipes calling for buttermilk tend to have their roots in the early 1800's before the introduction of baking powder.
Buttermilk is the left overs after you churn butter from CREAM. You can't churn butter from regular milk.
The yellowish or whitish solid of fat, water, and inorganic salts that is obtained by churning cream or whole milk is called butter.
- Encyclopedia Brittanica
My grandmother made her own butter and never skimmed/separated the cream from the milk. She just used the milk as it came from the cow. I've never done it, but that's how she described it to us. I know some people must skim the cream off and just use that, but from what I looked it, it does seem possible to do it from whole unhomoginized milk.
Thank you all for these explanations.
Heavy cream
To answer Bonjovibabe, I did a little enquiry: "babeurre" here in France is when you totally melt butter and let it solidify again. You then get the fat on the surface which you can scrape off. Not quite the same thing...
Buttermilk
If I consider what Narie is saying in her message, I don't really need to spend time trying to recreate buttermilk if I am using baking powder??
Is this really necessary to do the recipes (as stated among the ingredients) and obtain a good taste?? 
I see that Jopalis buys it and doesn't use it anyway!
My recomendation is to just check the baking aisle of your store or where ever it is that regular powdered milk is kept. Then poke around and look for a cannister or box of specifically powdered buttermilk. The reason I opt for the powdered buttermilk is that it stays good for 6 months in the fridge. If you buy fresh buttermilk like in a bottle, it of course goes bad in a week or so. Most of us don't do enough baking to use up the entire amount of fresh buttermilk in a week. That's why keeping the powdered buttermilk on hand is great! It's there whenever I need it and it doesn't go bad quickly.
I find that I get really good results with recipes that use buttermilk. I hope you can find some powdered buttermilk and give your recipe a try.
Oilili
Babeurre is the French-Canadian term for buttermilk.
We dont have buttermilk here in france. The best bet is to use whole milk and vinager or I sometimes mix yogurt with milk to thin it down a bit. Not the same taste but it creates the same results.
Whole cream is Crème Flourette over here.
If you need any more help just give me a PM.
à plus
Carla
Oilili- It is best to use sour milk if you don't have buttermilk. Part of the reason is to activate the soda, the other part is...? Well, I guess you would say taste or tenderness. American cooks have had baking powder for 150 years, and still they haven't abandoned the use of buttermilk or sour milk entirely. In fact, some recipes don't even mention buttermilk anymore, they just say sour milk and assume you will know how to sour milk. It is one of those Americanisms-- like a stick of butter.
Quote by @%username% on %date%
%body%