I wanted to use only ingredients mentioned in, “Brew Like A Monk”.
I love BLAM just as much as the next Trappist beer lover but don’t be a slave to exactly what is written in it. In fact, there is a lot “between the lines” in BLAM that isn’t spelled out. That’s how the book was written, not so much as a recipe tome, but as a guide and springboard for your own adventures in those styles.
I don’t think dark Candi syrups are appropriate if you want to clone Chimay’s Dubbel. Sure, I’ve tasted delicious Dubbel style beers with D90 and even D180, but at 15 SRM, you don’t need the dark syrup and frankly, I don’t think the flavors in those dark syrups are right for that beer. D45 would probably be a better choice and I agree with
@z-bob that 8-10% is probably the right number. Turbinado sugar is another excellent choice.
For malt, I’d say blend a few base malts, Pils/Pale or Pils/Vienna, and use a <20 °L cara malt for complexity. CaraHell, CaraRed, and CaraBelge are some of my favorites. CaraBelge in particular is a great malt and right in the ideal color range for this beer, not to mention it’s flavor wheel touches on many of the hallmark flavors of this beer.
Lastly, at 1.061 OG and 7%, you are going to want to make sure you have a very fermentable wort and a healthy fermentation. Chimay’s flavor profile is distinct and really driven by fermentation and attenuation. You want to make sure you avoid the trap of making this a high gravity beer and instead keep it around 1.060 and shoot for >85% attenuation.
Hope this helps.