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 ganache, place the chocolate, heavy cream and corn syrup in a medium heatproof bowl over a pot of simmering water. Stir the mixture using a wooden spoon until melted and smooth, about 5 minutes.
Grab each cupcake with its bottom, dip cupcake in the ganache to coat the top, let excess drip off. Place cupcakes in refrigerator for 10 minutes to set the ganache.
Garnish with sprinkles.