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 boiling water, 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 chocolate buttercream, beat butter for a few minutes in a standing mixer with the paddle attachment on medium speed. Add powdered sugar and turn the mixer on low, beat until the sugar incorporates with the butter. Add vanilla, heavy cream and cocoa powder, beat on medium speed for another 3 minutes, or until buttercream becomes light and fluffy. Transfer buttercream to a piping bag with a very tiny round tip.
To prepare cream cheese frosting, place cream cheese and powdered sugar in a mixing bowl. Beat until pale and fluffy, about 5 minutes. Transfer frosting to another piping bag with a grass tip or hair tip.
To assemble, start piping the cream cheese frosting on the top of the cupcake. Squeeze the bag, pull up and away when frosting strand is long enough, stop pressure and pull tip away.
Once the top is covered with cream cheese frosting. Piping chocolate buttercream on top in a round and wide circle in the middle of the cupcake. Keep piping rounds to make it look like a nest.