If there was ever a beer worthy of cloning, it's the Westy 12. It tastes great and is one of my favorite beers, but nearly impossible to get. You need to schedule a time to visit the monks at the Abbey of St. Sixtus in Belgium, and then you can buy a case. Or you can buy individual beers at the pub across the way from the abbey. Not an economical way to acquire this beer unless you live in the area. You can buy grey market beer on eBay, which is a less expensive than the trip to Belgium but it costs about $20/bottle the last time I checked.
So, I'm going to try brewing this beer again. I tried once before but I got too cute and caramelized my own sugar (just like the monks.) This time around I'll go with commercially prepared Candi Sugar. In looking at the recipes around the web that purport to be Westvleteren 12 clones, they mostly seem to be too dark, wandering into porter/stout country. That's not going to get you a credible Westy 12 clone. Knowledge is power, so what do we know? The monks haven't been overly forthcoming, but we do have some information.
The Fermentables: We can't confirm anything here, but the most likely scenario is:
Belgian Candi Sugar
And all in quantities that are unknown. The beer may also contain some plain sugar, but no specialty malts. We know that all the darkness and most of the character in the beer comes from the sugar rather than from specialty malts, but how that is accomplished is one of the monks secrets they are not willing to share. All that Michael Jackson and the book 'Brew Like a Monk' (BLAM) say is that "caramelized sugar" is used. BLAM also says that 15 to 20% of the fermentables in a Belgian Dark Strong can be sugar, which would be between 2.75lbs and 4lbs for a 5 gallon batch.
The Mash and the Boil: We don't know anything here. Decoctions and infusions are possible, we just don't know.
The Hops: Michael Jackson wrote that Northern Brewer was used for bittering (which has since been replaced by a hop extract) and Styrian Goldings and Hersbrucker are used for flavor. BLAM says the same. Amounts and schedules aren't known.
The Yeast: We have solid info on the yeast. Westvleteren uses yeast they get from Westmalle, and they pitch fresh on every batch. "A secular worker" makes the drive to get the yeast on brew day. This means White Labs 530 Abbey Ale Yeast or Wyeast 3787 Trappist High Gravity are going to be good choices.
The Ferment: According to Brother Joris, head brewer of Westvleteren, fermentation begins at 68°F and then rises to at least 82°F (pretty darn warm), even in the winter. After apparent attenuation reaches 76-80% he begins cooling the beer to 68°F. Westvleteren 12 spends four to six days in primary before lagering to clarify (crash cooling to 50°F) for 8-10 weeks. Bottle conditioning is done at 79°F and takes 12 days. They pitch additional sugar and yeast for this.
The Water: Westvleteren has a water profile that is not very desirable for brewing, and BLAM says that they treat their water because its high in bicarbonates, sodium, sulfate and chloride. The Chimay water profile has identified by BLAM as being desirable for a Belgian Dark Strong Ale, so this is worth considering:
Hope this helps. Please don't make another 'Westvleteren 12 Clone' that comes in at 40° Lovibond. Also, don't try to make one that follows the BJCP color guideline for Belgian Dark Strong. That's also off the mark. For what it's worth here is part of my tasting notes, which were probably skewed a bit by the small tasting glass that made the color seem a bit lighter than it should be:
Pours a cloudy dark amber with a big fluffy off-white head. Creamy medium-full body. Taste of pears and apples, almost to the point of being like cider, but with the toffee and caramel malt and some spice behind it.