In a large bowl, combine the light brown sugar, cocoa powder, cornstarch, cinnamon, chipotle powder, cayenne powder, and salt. Stir well.
Whisk in the milk until fully combined with the dry ingredients.
Pour the mix into a pot and bring the mix to a quick boil over medium heat. Continue to stir often using the whisk. Once boiling, continue to whisk for one minute.
Remove from heat and add in the butter and vanilla extract. Whisk well until the butter is melted.
Pour the mix into a bowl and lightly cover with wax paper. Place the bowl in the refrigerator and allow the putting to set (roughly 90 minutes to 2 hours).