To make a chocolate cocktail, heat half and half and milk in a saucepan over low heat until bubbles appear. Whisk in sugar, then add chocolate squares gradually, whisking to melt and thicken the liquid.
Remove from heat, add rumchata and Kahlua, and stir. In a separate bowl, whisk powdered sugar and milk to create an icing. Dip mug rims into the icing, coat in brownie brittle crumbs, and place a chocolate square in each mug
Pour the hot chocolate mixture evenly into the mugs and serve.