Mini Chocolate Ganache Cake

For the cake:
Preheat oven to 350 F. Grease and line a 9×13-inch sheet pan.

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. Pour batter into the prepared pan, and bake for 15 to 18 minutes. Transfer pan to a wire rack and let cake cool in pan for 15 minutes. Then invert it out of the pan onto a rack and let it cool completely.

For the buttercream:
Beat butter for a few minutes in the bowl of 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.

Use a 3-inch cookie cutter to cut out 12 cake circles. Sandwich together three cake circles with buttercream in between to create one mini cake. Place the four frosted mini cakes on a plate and put it into the refrigerator to chill.

For the ganache:
Place the chocolate, heavy cream and corn syrup in a medium heat-proof bowl over a pot of simmering water. Stir the mixture using a wooden spoon until melted and smooth, about 5 minutes.

Remove cakes from the refrigerator, and place them on a wire rack over a sheet pan to hold the ganache dripping. Pour and spread ganache over the top and sides of the cakes.

For the chocolate flakes:
Melt the chocolate in the microwave on 30 second heat bursts until melted and smooth then pour it onto a large sheet of parchment paper. Use an offset spatula to spread the chocolate in a thin and even layer. Let it dry completely. Peel away the parchment paper while breaking the chocolate into pieces.

Press chocolate flakes into the ganache, all around the sides of the cakes. Sprinkle gold crystal sugar on top as garnish. Preferably leave the cakes at room temperature and serve on the same day.