From the brewery:

After releasing our incredibly popular imperial brown ale in 2017, the next logical step was to age it. We’ve aged the beer in Heaven Hill Bourbon casks for 6 months then refermented it with Grade A maple syrup. Rich maple and vanilla aroma from the Bourbon wood will tempt your tastebuds before you even take a sip. When you do, you'll be treated to a rich, sticky mouthfeel with a rounded Bourbon flavour, and a sweet, dessert like finish that will leave you looking forward to your next taste.