Make Cupcakes
Preheat oven to 350°F // 180°C, and fill cupcake pans with paper liners.
In a medium sauce pan, combine butter and coffee over low heat. When butter has melted, remove from heat and add cocoa powder and chopped Mexican chocolate. Whisk until smooth and chocolate has melted.
Add brown sugar, vanilla and salt, whisk to combine, then add whole eggs, plus yolk, and whisk again.
Sift in the flour, baking soda and cayenne (if using). Whisk thoroughly to combine.
Using a scoop, portion cupcake batter in liners. Do not fill more than 2/3 full. *See note
Bake for 15 minutes, A tester inserted into the center of a cupcake should come out with a few crumbs attached to it.
Let cupcakes cool in the pan for 10 minutes before removing to a cooling rack to cool completely.
While cupcakes are cooling, make buttercream.
Make Buttercream & decorate cupcakes
Combine butter and half of the powdered sugar in a medium bowl.
Using an electric mixer, beat together until combined, then slowly add the remaining sugar.
Add salt, milk and vanilla. Beat for 4 - 5 minutes until very fluffy.
Use a piping bag fitted with a tip or a small knife to frost the cupcakes.
Decorate with sprinkles and edible sugar skulls.