I originally spied the technique on David Lebovitz's blog, and he recommends an oven temperature of 120 C. I traded one of the instant read thermometers I got for my birthday for an oven thermometer (it's not excessive to have three different types of kitchen thermometer, is it?) and given the sensitivity of chocolate I thought this would be a prime opportunity to break it out. Pleased I did, as before the oven thought it was up to temperature it hit 130 degrees, so I turned it way down. You just spread the chocolate out on a tray, then every 10 minutes take it out and stir it, to break up lumps and make sure it cooks evenly.
Not really... I spread it out to set and have broken it into little chunks, which will probably still be delicious in cookies, but they are a little bit bitter. As you can see it's pretty lumpy, too. Hmm.
So for round two I reduced the temperature even further - about 110 C according to the oven thermometer, or 90 C according to the dial. If you don't have an oven thermometer just err on the low side - it might take longer but that beats burning it!
Each time I took it out of the oven I scraped it all into the centre of the sheet, to make sure it was evenly mixed and smooth, then spread it out again. Here are the progress pics:
Colourwise, it is pretty close to peanut butter when it's done - get your peanut butter jar out for comparison if you need to as a tiny bit of extra cooking can turn it completely. With the second batch I stopped when I got to that peanut butter colour, but put a little of it back on the tray for another 6-7 minutes; this photo shows the difference between the stuff I'd taken out and what I put back in (the 'good' one is really pale - perhaps a bit overexposed but it's a good demonstration of contrast - that colour changes very quickly!). Again, the overcooked stuff was a bit bitter - that dramatic darkening of colour is not desirable!
The finished product will set firm, though it takes a lot longer than normal chocolate to do so (several hours at room temperature), but can be warmed back to a liquid consistency in the microwave or double boiler as you normally would. Now my challenge is to decide what to make with it - what recipe do you think would best benefit from this delicious caramelly goodness?
340g white chocolate, min. cocoa solids 28% (the Callebaut bits, available from Moore Wilsons, are 28%)
Preheat oven to 100-110 C (err on the low side if unsure of accuracy). Spread the chocolate (chopped into small pieces if from a block) over a clean baking tray. If you don't have a perfectly clean tray line it with baking paper as the chocolate will pick up anything on the tray. Bake for 40-60 minutes, stirring every 10 minutes for the first half hour, then every 5-6 minutes. Stop when the chocolate is the colour of peanut butter. Use liquid or spread on a tray and break into chunks to put in cookies. The set chocolate will keep for a while (at least a month or so) and can be melted as normal when required.