Preheat oven to 350F. Grease three 8-inch round cake pans, and line the bottoms with parchment paper.
In a large bowl, whisk the flour, baking powder, baking soda and salt.
In another bowl, whisk the boiling water and cocoa powder until smooth.
Beat butter and sugar together on medium speed in a bowl of a standing mixer until light and fluffy. Beat in eggs, one at a time. Beat in buttermilk and vanilla extract.
Reduce the speed to low and beat in 1/3 of the flour mixture, followed by half of the chocolate mixture. Repeat with another 1/3 of the flour mixture and the remaining chocolate mixture. Beat in the remaining flour mixture until just incorporated.
Divide the batter and pour into the prepared pans, smoothing the surface with a spatula. Bake 20 to 25 minutes or until a toothpick inserted into the cake comes out clean.
Place the cakes (in the pans) on a wire rack to cool. Then invert the cakes onto a greased rack. Cool completely before frosting.
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. Remove bowl from heat.
To assemble the cake, place one cake layer on your serving plate and spread with 1/2 cup ganache. Place another cake layer on top, and gently press down a little bit. Spread with another 1/2 cup frosting. Place the last layer of cake on top.
Pour ganache over the top and sides of the cakes. Use a spatula to even out the ganache on the cake. Use a spoon to create swoops and peaks.