To prepare cupcake, preheat oven to 350F. Grease muffin tin or line with muffin liners.
In a large bowl, mix together sugar, flour, cocoa powder, baking powder, baking soda and salt.
Crack in the egg, spoon in the sour cream, and pour in the oil and the vanilla. Mix with a wooden spoon until just combined.
Pour in the hot espresso, and slowly mix until smooth and liquidy.
Divide batter evenly among prepared muffin cups, about halfway full. Transfer muffin tin to oven and bake for 15 to 18 minutes. Transfer muffin tin to a wire rack and let cupcakes cool completely.
To prepare buttercream, beat butter for a few minutes in a standing mixer with the paddle attachment on medium speed. Add powdered sugar and turn your mixer on low, beat until the sugar incorporates with the butter. Add heavy cream and espresso, beat on medium speed for another 3 minutes, or until buttercream becomes light and fluffy.
Frost the cupcakes with the buttercream. Top with crushed candies.