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1
Line the bottom of 3 buttered 8-inch round (2-inch-deep) cake pans with wax paper, butter the paper, and dust the pans with flour, knocking out the excess.
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2
Into a bowl sift together the flour, the cocoa powder, the baking soda, the baking powder, and the salt.
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3
In a large bowl with an electric mixer beat together the sugar and the butter until the mixture is combined well, add the eggs, 1 at a time, beating well after each addition, and beat in the vanilla.
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4
Beat in the flour mixture alternately with the buttermilk in batches, beginning and ending with the flour mixture and beating well after each addition, divide the batter among the pans, smoothing the tops, and bake the layers in the middle of a preheated 350F.
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5
oven for 25 to 30 minutes, or until a tester comes out clean.
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6
Let the layers cool in the pans on racks for 10 minutes.
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7
Run a thin knife around the edge of each pan, invert the layers onto the racks, and let them cool completely.
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8
The cake layers may be made 2 days in advance and kept chilled, wrapped tightly in plastic wrap.
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9
In a heavy saucepan whisk together the sugar, the cornstarch, a pinch of salt, and the chocolate and whisk in the boiling water.
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10
Bring the mixture to a boil over moderate heat, whisking constantly, and simmer it, whisking, for 2 minutes.
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11
Whisk in the liqueur, transfer the mixture to a metal bowl, and with an electric mixer beat in the butter, beating until the butter is incorporated.
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12
Set the bowl in a larger bowl of ice and cold water and beat the frosting until it is light and holds soft peaks.
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13
On a cake stand arrange 1 cake layer, spread the top with some of the frosting, and top the frosting with a second cake layer.
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14
Spread the second layer with some of the remaining frosting and top the frosting with the remaining layer.
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15
Spread the side and top of the cake with the remaining frosting.
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16
The cake may be assembled 4 hours in advance and kept at room temperature.
