I haven't had this beer but having Mosher's book and looking at his other published recipes we could have a stab at it.
5 gallons, 75% efficiency (All-grain)
OG 1055 FG 1003 - The website states 6.4%ABV and at this gravity you really need to dry it out to reach that (addition of piloncillo and high attenuating English yeast)
8 lbs Maris Otter
0.5-0.75lb Melanoidin (may seem like a lot but Mosher likes to use lots of higher-kilned malt in his recipes)
~0.25lb dark malt of your choice (chocolate, roasted barley, black, etc)
at least 0.75 lb of piloncillo sugar
You'll most likely need to play with the specialty grain amounts. If you fear any roast, and again, I haven't had the beer, then add the dark grain at the end of your mash to extract mostly the color.
I don't know if there's any hop flavor or aroma, but with the ancho chiles I suspect there is not or very little. Use 1-1.1oz 9%AA Northern Brewer at 60 minutes to get your bitterness around 32+ IBU as the only hop addition.
Ancho chiles - check this blogsite out: http://1227brewing.blogspot.com/2011...cho-amber.html
One word of caution regarding sugar, which I have had some less than desirable experiences with. Sometimes larger amounts of simple sugar has caused my yeast to stop fermenting earlier than I wanted. It's possible that they preferred this rather than the more complex malt sugars and "got lazy". Pitch enough healthy yeast. Monitor fermentation progress if you add the sugar at the end of your boil. You could decide before brewing that adding the sugar later in the process, perhaps a few days after the yeast gets going, may help with reaching that dry FG. Otherwise, don't add the sugar and see what you get on your first try!