@Lavinia -
The answer is ... it's not that simple.
First off, whatever powdered sugar you get is probably going to have an anti-caking agent in it, often cornstarch. This will make it unusable for chocolate. If you do find a powdered sugar that is suitable it may still be too coarse (the particles are too big), and you will still be able to feel the sugar crystals as grit on the tongue. Another thing to consider is that adding powdered sugar into chocolate will make it very thick - especially if you add it all at once.
A better process would be to take your own sugar and refine it in a food processor and then grind it into the melted liquor in a grinder/refiner. You can add cocoa butter to get to the fluidity you need. How much will depend on the amount of sugar you add.
Any tempered chocolate that is left over can be poured into a pan to let cool and to use for later remelting and retempering. If you need seed chocolate, pour some of the tempered chocolate into a thin layer - making sure it stays in temper. You can break this up and use it for seed in future batches. The rest of the chocolate can be stored at room temps, just make sure no moisture condenses on the chocolate. In other words, you don't need to put it in the refrigerator or freezer.
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clay - http://www.thechocolatelife.com/clay/
