Preheat oven to 350F. Grease three 5-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 half of the flour mixture, followed by the 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 15 to 20 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. Then slice cake layers in half horizontally to make 6 cake layers.
To prepare frosting, beat butter for a few minutes in a standing mixer with the paddle attachment on medium speed. Add powdered sugar and vanilla, turn mixer on low, beat until the sugar incorporates with the mixture. Then beat on medium speed for another 3 minutes, or until buttercream becomes light and fluffy. Add mascarpone cheese and beat on medium speed until combined, about a minute.
To assemble the cake, place one cake layer on a serving plate, spread 1/4 cup frosting on top, then continue until the last cake layer is placed. Frost the cake top and sides with the remaining buttercream. Chill cake in refrigerator.
To prepare ganache, place chocolate and heavy cream 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 from heat and let cool slightly.
Pour the ganache over the chilled cake. Use a spatula to quickly push the ganache over the edges of the cake and let it drip down to create the drip effect.