Can I use tartaric acid in making Snickerdoodles? I am following the Cook's Illustrated recipe for Snickerdoodles, which calls for a 2:1 ratio of cream of tartar to baking soda. They state explicitly at the very top of the recipe:

Cream of tartar is essential to the flavor of these cookies and it
works in combination with the baking soda to give these cookies a
lift. Do not substitute baking powder.

Unfortunately, I'm in Sweden and supermarkets do not sell cream of tartar, only tartaric acid (vinsyra).
Previously I've simply substituted baking powder (against their recommendation), but now I am wondering if would be possible to use that tartaric acid to get the &quot;characteristic tangy flavor&quot; that they describe. If so, how?
I've never tried it, but you might try 1/2 the amount of tartaric acid based on information from Nigella Lawson :

Cream of tartar is made by combining tartaric acid with potassium hydroxide. This partially neutralizes the tartaric acid, so cream of tartar is less acidic than tartaric acid.
If the tartaric acid is used in baking or added to egg whites before whisking into meringues then it should be possible to use cream of tartar in roughly double the quantities of the tartaric acid, though we have not tested this. However we would not recommend using cream of tartar as a substitute in any other recipe.