Pour 1 cup (8 ounces or 240 mL) of the plant-based milk into a small or medium saucepan. Add the ginger, cinnamon, nutmeg, allspice, cacao powder, maple syrup, and molasses. Bring to a rapid simmer over medium heat, whisking occasionally to incorporate the cacao powder and spices into the milk.
Meanwhile, whisk together the remaining 1/2 cup (4 ounces or 120 mL) of the plant-based milk with the arrowroot powder in a bowl until no clumps remain.
Once the hot chocolate is at a rapid simmer, pour the arrowroot slurry into the hot chocolate, whisking constantly until the mixture thickens, just 30-45 seconds. If you double the recipe, it will take a bit longer for the hot chocolate to thicken. Remove from heat and serve warm.