For steeped grains plus malt extract:
BJCP 26B Belgian dubbel description: "Characteristic Ingredients: Spicy-estery Belgian yeast. Impression of a complex grain bill, although many traditional versions are quite simple, with caramelized sugar syrup or unrefined sugars and yeast providing much of the complexity. Continental hops. Spices not typical; if present, should be subtle."
Vital statistics: OG: 1.062 – 1.075, FG: 1.008 – 1.018, IBUs: 15 – 25, SRM: 10 – 17, ABV: 6.0 – 7.6%
Pilsner malt should be used for a base malt. Belgian pilsner malts are preferred, but German and other continental European malts are acceptable. Dark crystal malts, such as 80L to 120L, are important for dark color and raisin-like flavors. Other possible specialty malts are aromatic, melanoidin, cara-Munich, special B, and biscuit. The sources caution against excessive specialty malt complexity.
Sugar additions are 5 to 10% of the fermentables with the aim of lowering the final gravity. Belgian candi sugar or syrup (D-90) is a top choice.
A potential problem in beers with high sugar levels is an inefficient fermentation and high final gravity. One way to address this problem is to not add the sugar during the boil. Let the fermentation proceed normally, then add the sugar when the fermentation has finished.
The preferred hops are Styrian Goldings and classic noble hops, such as Hallertau, Tettnang, and Saaz. A single bittering addition is sufficient because hop flavor and aroma are a distraction in this style. The BU/GU ratio target is .3.
Use a Belgian yeast that tolerates high gravity. Monastic or abbey yeasts might be the most authentic. Belgian practice is to pitch the yeast around 65F (18C) then allow the temperature to freely rise to 75 or even 80F (24C to 27C).