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 the assembling, place graham cracker crumbs in a small bowl, and whip heavy cream to stiff form.
Smear a layer of Nutella on top of the cupcake. Dip cupcake in the graham cracker crumbs to coat the top, followed by a dollop of whipped cream. Insert a banana slice and a graham cracker cookie. Sprinkle chocolate shavings and drizzle with dulce de leche.