Whisk together brown sugar, cocoa, cornstarch, cinnamon, and 1/8 tsp salt in a heavy medium saucepan, then whisk in almond milk. Bring to a boil over medium heat, whisking often, then boil, whisking, 1 minute. Remove from heat and whisk in butter and vanilla. Chill in a bowl, surface covered with a piece of buttered wax paper, until cold, at least 1 1/2 hours.