Trouble tempering Valrhona Couverture Manjari (64%) Chocolate I have tried to temper equal amounts of Valrhona Couverture Manjari (64%) chocolate + baking chocolate (Van-Heusen brand, dark chocolate) without any success. 

Used the seeding method &amp; followed the instructions given by Valrhona, ie melt @ 131-136F, cool to 82-84F, reheat to 88-90F. Working with a very small batch (~400g), used a bain-marie, IR thermometer. 

The chocolate is nice and shiny but simply does not harden at room temperature. 

The instructions provide by Valrhona say it needs to be melted for 12h, but I have read articles where others have been able to temper it without melting for 12h. 

I have no clue what's going wrong. Would be grateful for any guidance.

Thanks in advance,
Asha
A couple of thoughts...

Not commenting on Valrhona specifically but 12h at 131F seems like a very long time at a very high temp. Can you temper it on its own at a lower temperature? Cooling the chocolate to 84F without contact to a cold(er) surface won't seed it well. You can also try adding rough shaved tempered chocolate in sufficient quantities during the 82F-84F state.

What is the cocoa butter content of the Van-Heusen? I don't know anything about it but if it is extremely low, you may be creating a low cocoa butter chocolate that is difficult to temper.

Take all of this with a grain of salt (please don't add any). I worked in chocolate for 10 years but haven't for the past 15 and I may be losing my mind. These are the steps I would take if it were up to me.