Preheat the oven to 175°C.
Butter one 4 x 3-inch round cake pan, one 6 x 3-inch round cake pan and one 8 x 3-inch round cake pan. Line the bottoms with parchment paper, and butter and flour the pans.
Sift the flour, cocoa, baking soda, and salt together in a medium bowl.
In the bowl of an electric mixer fitted with a paddle attachment, cream the butter and sugars on high speed until light, approximately 5 minutes.
Add the eggs and vanilla and mix well. Combine the buttermilk, sour cream, and coffee.
On low speed, add the flour mixture and the buttermilk mixture alternately in thirds, beginning with the buttermilk mixture and ending with the flour mixture. Mix the batter only until blended.
Divide the batter between the three pans and smooth the tops with a spatula.
Bake on the middle rack of the oven as follows: for the 4-inch pan 45 to 50 minutes, or until a toothpick comes out clean; for the 6-inch pan 70 to 75 minutes, or until a toothpick comes out clean; and for the 8-inch pan 85 to 90 minutes, or until a toothpick comes out clean.
Cool the cakes in the pans for 10 minutes each on a cooling rack, carefully remove from the pans and allow to finish cooling.
For the frosting, place the chocolate chips and heavy cream in a bowl set over a pot of simmering water, stirring occasionally, until the chips are completely melted.
Off the heat, add the corn syrup and vanilla and allow the chocolate mixture to cool to room temperature.
In the bowl of an electric mixer fitted with the whisk attachment, whisk the chocolate mixture and softened butter on medium speed for a few minutes, until it's thickened.
Place a dollop of the frosting on a serving plate to hold the bottom cake layer in place.
Place the 8-inch cake on the plate, flat side up. Frost the top and sides of that layer. Place the 6-inch cake in the center of the top of the 8-inch cake, flat side up, and frost the top and sides.
Place the 4-inch cake in the center of the top of the 6-inch cake, flat side down, and frost the top and sides.
Pipe any remaining frosting decoratively around the cake and decorate with the candy-coated chocolates.