For two servings in large mugs: Put a small amount of margarita mix in a shallow dish. Same with the walnuts. Dip the rims of the two mugs in the margarita mix, then coat rims with walnuts. Set aside.
In a small saucepan over low heat, put milk and broken-up candy bars. Heat, stirring occasionally until chocolate is melted and milk is just starting to boil. Remove from heat and pour into mugs. Add bourbon. Top with lots of whipped cream. What a treat!
Note about whipped cream: I prefer whipping my own. Using electric mixer, in a small, deep bowl, whip 1/2 cup of heavy cream until it is thickened, but still soft (I don't like firm whipping cream in my cocoa). For this recipe, as it's thickening, add a teaspoon or two of sugar until it's good and sweet.
Also, if you ever whip your own cream and have some leftover, put it in a small colander placed over a bowl or pan and refrigerate, uncovered. It will last a day or two without getting runny.